DAVinSON — SCOTTISU CARBONIFEROUS BRACHIOPODA. 
223 
has not been ^hitlicrto completely investigated, we wiU give figures of the in- 
terior of the valves, for the sake of explaininjj the more recent but provisional 
interpretation and names tliat 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 giveu will liowever sufiBce 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 iuformcd 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 p is due to what may be termed the ventral ad- 
justors, that these muscles form a sear close to the outer border of the divari- 
cator in the ventral valve. The other extremities of this muscle converge and 
pass rouud 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 adjusters (?), 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 
border of the ocelusors. The brachial muscle has both its extremities attached 
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 miiscles, which perhaps may be named the 
brachial. The mesenteric («) 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- 
preseiTcd examples, which may be dredged alive close to some portions of the 
Irish and Scottish shores. The oral arms are thick, fleshy, and spirally coiled ; 
the volutions are few, and dii-eeted vertically towards the cavity of the dorsal 
valve, somewhat as is seen in Diseina 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 
difi^erent from that of Brachiopoda generally, but as stiU conformable to it in 
being penetrated by canals wluch are prolonged from the lining membrane of 
the shell, and which pass towards its external surface, these differing, however, 
from Terebratulae in not arriving at that surface, and in breaking up into 
minute subdivisions as they approach it. 
XLIV.— Crania quadrata. M'Coy. PI. v., figs. 12-21. 
Orbicula quadrata. M'Coy, " Synopsis of the Carboniferous Possils 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 shght 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 
