therefore, be any doubt but that these structures are primitively hollow, though their central cavities often appear to become filled up by a secondary deposit of sclerenchyma, as growth

proceeds. In this primordial hollowness of the spines is to be found, I believe, the real clue to their nature; and I can hardly doubt that instead of being merely appendages of the corallum, they are truly of the nature of peculiarly modified zoöids or corallites. The correctness of this view is most readily recognised when we come to examine thin sections of those forms which have usually been separated from Monticulipora under the generic title of Dekayia, E. and H. In these cases (Pl. XV., figs. 1 b, I c) the supposed spines are very much reduced in number, but they are quite exceptionally developed, and they constitute the well-known surface-projections, which are characteristic of the genus. These surface-projections certainly seem to be imperforate at their apices, but thin sections demonstrate conclusively that they are hollow internally, and that they only differ from the ordinary corallites in the greater thickness and density of their walls and the apparent absence of tabulæ. I do not myself entertain any doubt as to these being a peculiar form of corallites-doubtless tenanted in life by peculiar zoöids--the mouths

of which became closed by secondary deposit as the corallum assumed its final characters. Nor have I any doubt that the spines of forms like M. moniliformis, Nich. (figs. 36, 37), M. Jamesi, Nich., M. tumida, Phill., M. gracilis, James, and others, are similarly peculiarly modified corallites, the mouths of which become finally closed. A further evidence of this is to be found in such species as M. frondosa, D'Orb., in which the spines do not appear as spines upon the surface, though thin sections exhibit appearances precisely similar to what has been indicated as occurring in the forms alluded to above. On the contrary, the spines remain permanently open, and appear on the surface as minute thickened apertures between the ordinary calices, so that they have been both recognised and figured as a special group of corallites (Ann. and Mag. Nat. Hist., ser. 4, vol. xviii. p. 92, Pl. V., fig. 11). Lastly, if we admit the probable correctness of the views here advanced, we have a very interesting analogy established between certain forms of Monticulipora and some of the species of Stenopora, Lonsd., in which structures of a precisely similar nature occur. Thus, if we examine a tangential section of Stenopora Tasmaniensis, Lonsd., taken just below the surface (fig. 38), we see that the surfacespines are continued inwards precisely as they are in Monticulipora moniliformis and allied types, while they are similarly composed of concentrically-disposed lamellæ of dense sclerenchyma. The central cavities of the spines seem, however, to be more or less completely obliterated with age; and the corallites in the outer portion of their course (fig. 38, A) exhibit the annular thickenings of their walls which are so characteristic of the genus Stenopora. In spite of these differences, the resemblance between the spines of the Monticulipora above alluded to and the similar structures in certain species of Stenopora is so striking that one can hardly resist the conviction that there must subsist between the two a relationship of real affinity.

More conspicuous, more generally familiar, and more uniformly present than the spines, are those structures in the Monticulipora, which are known as "monticules" and "macu

læ." The "monticules" or "mamelons" are circumscribed areas on the surface of the corallum, which are, typically, elevated so as to form a series of rounded, oval, or elongated

Fig. 38.-A, Vertical section of a few of the corallites of Stenopora Tasmaniensis, Lonsd., in the final portion of their course, enlarged twenty times, showing the annular thickenings of the tubes and the remote tabulæ; B, Tangential section of the same, taken just below the surface, similarly enlarged, showing the transversely divided spiniform corallites between the ordinary tubes. Carboniferous, Australia.

projections, but which may be nearly or quite level with the general surface. Sometimes the "monticules" are composed of corallites which differ in no conspicuous feature from those which form the rest of the coral (e. g., in M. ramosa, D'Orb.); in other cases (e.g., M. pulchella, E. and H.) the corallites which form the "monticules" are of a markedly larger size than the average; while in still other forms (e.g., M. frondosa, D'Orb.) the "monticules"-in this case flat-are occupied by corallites much more minute in their size than those which form the bulk of the colony. The so-called "macula" are simply "monticules," in which the mouths of the tubes have become closed by a calcareous membrane.

The only other point as regards the structure of the Monticulipora which need specially be noticed here concerns the disposition of the tabula. These structures are invariably present in the typical Monticulipore, and are almost invariably "com

plete." The only exception to the latter part of this statement with which I am acquainted is the curious M. frondosa, D'Orb., in which the tabulæ seem to have the form of crescentic diaphragms, with an excentric aperture or perforation on one side. As a rule, the tabulæ are very sparsely developed in the axial and deeper portions of the corallum, and become much more numerous as the tubes approach the surface. As a rule, also, the tabulæ are conspicuously more numerous and more closely set in the smaller corallites of the two sets of tubes of which the corallum is normally composed. In a few forms, lastly, the tabulæ may become curved, so as to assume a subvesicular or vesicular aspect; but this is quite an exceptional feature.

With regard to "septa" in the Monticulipora, it need only be said that no traces of these structures have hitherto been detected in any Monticuliporoid. Occasionally one may see in thin transverse or tangential sections a single azygous septum projecting for some distance into the interior of a tube, but this seems to be really the result of fission of a corallite, as we have seen to be the case in Chatetes.

As regards the development of the Monticulipore I have little to say from actual observation, and that little will be best said. after I have given some account of the views held upon this subject by Dr Gustav Lindström (Ann. Nat. Hist., ser. 4, vol. xviii. p. 5 et seq.) As I find myself in this matter unable to accept the conclusions of the distinguished Swedish palæontologist, it is only just that I should quote his account of the development of Monticulipora at length. Upon this point he remarks:

"If numerous specimens of the common M. petropolitana, Pand., be closely scrutinised, it will be seen that its semi-globose colony, so closely resembling a Favosites in its initial development, has an origin that could hardly be suspected. It begins, indeed, as a Bryozoön, as a Discoporella, as what Hall has termed Ceramopora imbricata (Pal. N.Y., vol. ii. p. 169, Pl. 40 E, figs. I a-1 ). There can be no doubt that this is closely allied to the recent Discoporella (see Fr. Smitt, Öfv. Vet. Akad.

Förhand. 1866, p. 476, Pl. XI., fig. 4). The basal surface of a Monticulipora, when the epitheca is very thin, clearly shows that it is in its first origin a Ceramopora. The smallest Ceramopore which I have hitherto seen consist of a thin circular disc with elevated edges. From the smooth centre of the superior surface four or five wedge-shaped zoœcia radiate outwards, each of a length of 1-5th millim., their mouths being oblique, with the inferior lip somewhat protracted. On both sides of the mouth there is a short, pointed spine. In its interior such a zoœcium is transversely divided by some irregular tabulæ. The interstitial tubes which are so characteristic of the Discoporellida are also distinctly seen between the zoœcia of Ceramopora. New zoœecia are budded forth in quincunx from the corner of the old zoœcia, and in the periphery of the colony they become more crowded, having the mouth oval and erected. In the interstices is seen what might be taken for a cœnenchyma; but this in reality is composed of nothing but smaller irregular zoœcia. When the colony has spread out laterally, there are seen at the sides of the first smooth centrum several others regularly distributed on the surface, from which zoœcia radiate just as if the disc were composed of an aggregation of coalescent initial buds. When the colony has thus gained the expanse of an inch or more, the zoœcia grow vertically upwards, and the colony by-and-by assumes a semi-globular shape, and is converted into a Monticulipora. All the zoœcia are then tubular, their mouths quite circular, and armed with a pair of very short spines, their size varying in different cases. The larger zoœcia have around them either an empty space, or, as above stated, a cellular tissue resembling a conenchyma, and consisting of smaller circular or polygonal tubes. The walls of the zoœcia are solid, without any perforations, and interiorly quite smooth. and destitute of projecting ridges or septa. The tabulæ are very irregular in the large tubes, being oblique or deeply sunk in the wall; in the narrower tubes they are dense and regular. The large zoœcia are clustered in groups at tolerably regular intervals, each group of six or eight members. In Upper