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In order to avoid the use of double terms, I propose to call them "hydrospires," and their apertures, "pores,” “fissures,” or "spiracles" according to their form.

In Caryocrinus ornatus the hydrospires (fig. 4,) are of a rhomboidal form, and have each of the four sides bordered by a single row of small tubercles. Some of these tubercles have a single pore in the summit, while others are perforated with a variable number,-from two to twenty, or perhaps more,thus becoming vesicular or spongy. It is only the apex of the tubercle, however, that has this structure, for, when this is worn off, there is only a single pore to be seen. The pores penetrate through the plates but do not communicate directly with the general cavity of the body. Internally each hydrospire consists of a number of flat tubes arranged parallel to each other and lying side by side, in the direction of the dotted lines in fig. 4, a. Each tube receives two of the pores seen on the exterior-one pore at each end. These tubes are composed of a very thin shelly membrane, which although possessed of sufficient rigidity to maintain its form, was no doubt of such a minutely porous texture as to admit of the transfusion of fluids in both directions, outward and inward. In a large hydrospire there are about twenty of those tubes. Their greatest breadth is at their midlength where they are crossed by the suture c, c; and as they become narrower accordingly as their length decreases, the one in the middle projects the deepest into the perivisceral cavity. In consequence of this arrangement when a section is made across the hydrospire at the suture c, c, fig. 4a, the form b, is obtained where c, c, is the surface of the shell, while the comb-like structure below represents the tubes.

Specimens of C. ornatus almost entirely empty are often found and in some of these the internal form of the hydrospires is sometimes preserved. Those that I have seen have the form of small rhomboidal pyramids, with four slightly convex sloping faces, and composed of a number of vertical parallel plates-the casts of the interior of the tubes-the substance of the tube itself not being preserved. I have, however, several polished transverse sections, in which I think the thin walls can be seen.

The structure of the hydrospires is such, that there can scarcely be any doubt that they are respiratory organs. The sea-water entered through the pores, and ærated the chylaqueous fluid, contained in the perivisceral cavity, by transfusion through the exceedingly thin membranous shell, that composed the walls of the tubes. The number of pores varies with the size of the individual. In large specimens these are from 800 to 1000.

It has been stated by some authors that the pores were passages for the protrusion of internal organs connected with the vitality of the animal. The fact, however, that the pores do not penetrate into the general cavity of the body, disproves this theory; and, moreover, through many of the tuberclesthose with a vesicular and spongy summit, such protrusion would be utterly impossible.

In Caryocrinus ornatus there are thirty hydrospires arranged as follows:

1. Ten at the base-half of each on a basal plate and the other half on one of the subradials, their longer diagonal vertical.

2. A zone of six around the fossil at the mid-height-their longer diagonals horizontal. These seem to be imperfectly developed, for, on the inside, the tubes occupy only a small space in the center.

3. A third band of fourteen-two of them with their longer diagonals vertical and the others arranged in six pairs, the diagonals of each pair inclining toward each other, upward, at an angle of about 30°. There are only three interradii in Caryocrinus: the mouth is placed in one of them and the two hydrospires with vertical diagonals in the other two.

In Pleurocystites the hydrospires are also of a rhomboidal form, but instead of having the tubular structure of Caryocrinus, they consist of a number of parallel inward folds of an exceedingly thin part of the shell. These folds no doubt represent the tubes of Caryocrinus. If we grind down a hydrospire of this latter, so as to remove all the shell, and expose the edges of the tubes, it then exhibits precisely the same form as fig. 5 a, i. e., the form of a rhomb, longitudinally striated at right angles to the suture, and with no pores. The transverse section in Pleurocystites only differs from that in Caryocrinus in having no shell between the points c, c. In the hydrospire of Pleurocystites robustus, of the Trenton limestone, we have the commencement of the formation of an internal gill with a single spiracle. The surface is not flat, as it is in many species, but concave as shown in the section; and it is evident that if the concavity should be carried further, and at the same time the points c, c, made to approach each other, the effect would be to produce an elongated sack, deeply folded on one side, and with a fissure extending the whole length on the other side. The transverse section of such a sack would be fig. 6, the same as in Pentremites. Again, if we contract the four sides, gradually curving them outward at the same time, but not diminishing the superficial extent of the walls of the folds although altering the form to corres

pond with the decreasing aperture, the result would be a deeply folded, flask-shaped sack, with a small round orifice like fig. 7, which is the internal gill of a spider.

In Palæocystites tenuiradiatus, a species very characteristic of the Chazy limestone, the whole surface (in the condition in which the fossil is usually found) is covered with deeply striated rhombs, the fissures being deepest where they cross the suture and growing gradually shallower as they approach the center of the plates, where they die out altogether. Detached plates occur in vast abundance but no perfect specimens have ever been found. I discovered, however, several fragments of the body sufficient to give the general form and to show that, when the surface is perfect, all these fissures are completely covered over by a very thin shell, and that, when they cross the suture there is a small pore in the bottom of each which penetrates to the interior. The rhombs of this species are thus external hydrospires. The fissures seen in the ordinary weathered specimens are the remains of flat tubes like those of Caryocrinus, situated on the outer instead of the inner surface of the test. The chylaqueous fluid passed outward through the pores and filled the tubes, to be ærated through the thin external covering by the surrounding water. In Caryocrinus the water passed inward, through the pores, into the tubes and rated the fluid within the general cavity of the body.

The discovery that the fissures and pores of the Cystidea, do not communicate directly with the general cavity of the body is entirely due to Mr. Rofe. After reading his highly important paper, I re-examined a great number of specimens and found sufficient to confirm his observations.

3. On the genus Codaster.

Every author who has described a species of this genus has remarked the peculiar striated areas in the interradial spaces. Prof. McCoy, the founder of the genus, pointed out their resemblance to the hydrospires of the Cystidea, but it was Mr. Rofe who first showed that they were also identical in structure therewith. On comparing one of those with that of the cystidean Pleurocystites, fig. 5, we at once perceive that they are the same in the external form while Mr. Rofe's figures show that the section at d, d, has the structure of fig. 9, which only differs from fig. 5 b, in being straight above instead of concave, and in being divided into two parts. This division is the result of the position of the arm which cuts the hydrospire in two, in a direction parallel to the fissures. By drawing the points d, a, and a, d, together we get figure 10, which

is, in general plan, a section across one of the ambulacra of a Pentremite. On examining nearly all the published figures of species of this genus I find that there is a series of forms which exhibit a gradual passage, from those with the hydrospires almost entirely exposed, as in fig. 8, through others in which they are crowded more and more under the arms, until at length they become altogether internal.

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Fig. 8. Summit of C. acutus McCoy, m, v, mouth and vent; d, d, suture across the posterior hydrospire. 9. Section across the hydrospire from d, to d, at a, is the place of the arm. 10. The section contracted as in fig. 6. 11. Summit of Pentremites caryophyllatus De Koninck.

In C. acutus, fig. 8, only a small portion of the hydrospire is concealed under the arm. In C. Canadensis, a new species

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lately discovered in the shales of the Hamilton group in Canada West, each of the four interradial spaces, in which the hydrospires are placed, is excavated, in such a manner as to form a small triangular pyramid, with two of its faces sloping down toward the sides of the two adjacent arms. these two slopes are placed the hydrospires, which appear to have one fissure entirely under, and another partly under the arm, five others being fully exposed. S. S. Lyon has described a species under the name of C. alternatus in the "Geology of Kentucky," vol. iii, p. 494, from the Devonian rocks of that State, which closely resembles C. Canadensis, but is still distinct therefrom. Speaking of the structure of the summit he says: "the depressed triangular intervening spaces are filled with seven or more thin pieces, lying parallel to the pseudambulacral fields, articulating with the summit of the second radial, and the prominent ridge lying between the pseudambulacræ. These pieces were evidently capable of being compressed or depressed: the "point" at the lateral junction of the second radials is in some specimens folded over toward the mouth so as to entirely obscure these triangular spaces by covering them." This important observation proves that even in the same species the hydrospires may be

either partly or wholly concealed under the arm. The "point" to which Mr. Lyon alludes is seen above, in fig. 11, just below the letter b. It is the same as the "small triangular pyramid" in C. Canadensis. It is evident that (supposing the shell to be flexible) if these points were to be drawn inward, the movement would gradually cause what remains exposed of the hydrospire to be covered until at length it would be entirely concealed under the arm. The five points would then be situated in the angles between the five ambulacra as they are in the genus Pentremites, fig. 15. The concealment of the hydrospires may also be the result of the widening of the arm. This is well shown in P. caryophyllitus DeKoninck, (P. Orbignyanus according to Roemer,) P. Schultzii De Ver., and several other species. In these the apices of the pyramids remain near the margin, but the hydrospires are nearly covered by the wide arms. This is shown in fig. 11, where the ends of the fissures of the hydrospires are seen along the sides of the angular ridges which extend from the apices of the pyramids to the angles between the arms. I do not think that such species can be referred to Pentremites, and if I had specimens before me instead of figures only, I would most probably institute a new genus for their reception.

Our specimens of C. Canadensis are well preserved and show the characters of the arms perfectly. After many careful examinations under the microscope, I can state positively that in this species the so-called "pseudambulacral fields" have no pores. The markings that have hitherto been mistaken for ambulacral pores in Codaster are not pores, but the small pits or sockets which received the bases of the pinnulæ. The rays therefore in this genus are not " pseudambulacral fields," in the sense in which that term is used in descriptions of species of Pentremites, but simply recumbent arms, identical in structure with those of the cystidean genera Glyptocystites, Callocystites, Apiocystites, and others. They lie upon the surface of the plates which constitute the shell of the animals-not imbedded into them as in Pentremites. The large lateral aperture is both mouth and vent, and the central opening heretofore called the mouth is the ambulacral or more properly, the ovarian orifice. As, therefore, Codaster has the arms of Apiocistites, the hydrospires of Pleurocystites and the confluent mouth and vent common to all Cystideans, I propose to remove it from the Blastoidea and place it in the order Cystidea.

4. On the genus Pentremites.

• In Pentremites the hydrospire is an elongated, internal sack, one side of which is attached to the inside of the shell while

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