DAVIDSON — SCOTTISH CAEBONIFEROUS BRACHIOPODA. 



228 



has not been hitherto completely investigated, we will give figures of the in- 

 terior of the valves, for the sake of explaining the more recent but provisional 

 interpretation and names that have been applied to the muscular impressions by 

 Mr. Hancock. But we must hasten at the same time to observe that the 

 interior appearance and shape of the muscular and other impressions are 

 very different in detail in certain species, although very similar in others. The 

 figures here given will however suffice to explain the general character. 



Mr. Hancock, who at my request in May 1859 examined the animal of three 

 or four badly preserved specimens of C. anomala (the only specimens then to 

 be procured), has informed me that the impressions a are undoubtedly due to 

 the occlusor, r to the divaricators, and that when the former muscles relax 

 and the latter contract, the fluid in the perivisceral chamber will be forced for- 

 wards, and thus the valves will be opened a little in front, the action being the 

 same as in Lingula ; that v due to what may be termed the ventral ad- 

 justors, that these muscles form a scar close to the outer border of the divari- 

 cator in the ventral valve. The other extremities of this muscle converge and 

 pass round the outer margin of the occlusor, to which they adhere ; but Mr. 

 Hancock could not exactly determine how they terminate, d p are considered 

 due to be the dorsal adjustors (?), one end of the muscle being attached to the 

 dorsal valve, close to the outer border of the divaricators, the other most pro- 

 bably to the anterior process of the ventral valve ; although this could not be 

 satisfactorily determined, from the indifferent state of preservation of the speci- 

 mens, at any rate the fibres of this extremity were firmly united to the inner 

 b order of the occlusors. The brachial muscle has both its extremities attache d 

 to the same valve (the dorsal) — the anterior end to the ventral process, the 

 dorsal close to the outer margin of the occlusor, with which it blends its fibres ; 

 that the arms are fixed to these muscles, which perhaps may be named the 

 brachial. The mesenteric {n) is a flat thin membranaceous muscle, binding the 

 dorsal mysentery to the process of the hinge-margin, to which, according to 

 Mr. Woodward, the cardinal muscle is attached ; but we may hope that before 

 long Mr. Hancock will have been able to investigate anatomically some well- 

 reserved examples, which may be dredged alive close to some portions of the 

 rish and Scottish shores. The oral arms are thick, fleshy, and spirally coiled ; 

 the volutions are few, and directed vertically towards the cavity of the dorsal 

 valve, somewhat as is seen in Discina and other genera. We may also notice 

 that the brachial muscle is very closely united to the occlusor ; that it is diffi- 

 cult to distinguish the two in the generality of specimens. 



Dr. Carpenter has stated the structure of the shell in this genus to be widely 

 different from that of Brachiopoda generally, but as still conformable to it in 

 being penetrated by canals which are prolonged from the lining membrane of 

 the shell, and which pass towards its external surface, these differing, however, 

 from Terebratulse in not arriving at that surface, and in breaking up into 

 minute subdivisions as they approach it. 



XLIV.— Ceania quadkata. M'Coy. PI. v.; figs. 12-21. 



Orbicula quadrata. M'Coy, " Synopsis of the Carboniferous Eossils of Ire- 

 land," p. 104, pi. XX., fig. 1. 1844. 



This species varies much in shape, on account of its mode of attachment, 

 which is by the entire surface of its lower valve ; but when quite regular, is 

 marginally sub-quadrate, almost circular, or slightly elongated, oval: the 

 posterior edge being usually straight, or with a slight inward curve, while the 

 shell is at the same time wider anteriorly than posteriorly. The upper or free 

 valve is conical, or limpet-like, the vertex being sub-central and closer to the 

 posterior than to the anterior margin. Externally the surface is marked with 



