BRACHIOPODA. 305 
recently by Dr. Carpenter; according to this last observer, it 
consists of flattened prisms of considerable length, arranged 
parallel to each other with great regularity, and obliquely to the 
surfaces of the shell, the interior of which is imbricated by their 
out-crop. This structure is found only in the Rhynchonellide ; 
but in most—perhaps all the other Brachiopoda—the shell is 
traversed by canals from one surface to the other, nearly verti- 
cally, and regularly, the distance and size of the perforations 
varying with the species (vol. i, t. 1, f. 5). Their external 
orifices are trumpet-shaped, the inner often very small; some- 
times they bifurcate towards the exterior, and in Crania they 
become arborescent. The canals are occupied by ceecal processes 
of the outer mantle-layer, and are covered externally by a thick- 
ening of the epidermis. Mr. Huxley has suggested that these 
coeca are analogous to the vascular processes by which in many 
ascidians the tunic adheres to the test; the extent of which 
adhesion varies in closely allied genera. The large tubular 
spines of the Productidze must have been also lined by prolon- 
gations of the mantle; but their development was more probably 
related to the maintenance of the shell in a fixed position, than 
to the internal economy of the animal.—Kine. Dr. Carpenter 
states that the shell of the Brachiopoda generally contains less 
animal matter than other bivalves; but that Discina and Lingula 
consist almost entirely of a horny animal subtance, which is 
laminar, and penetrated by oblique tubuli of extreme minute- 
ness. He has also shown that there is not in these shells that 
distinction between the outer and inner layers, either in struc- 
ture or mode of growth, which prevails among the ordinary 
bivalves ; the inner layers only differ in the minute size of the 
perforations, and the whole thickness corresponds with the outer 
layer only in the Lamellibranchiata. The loop, or brachial pro- 
cesses, are always impunctate. Mr. Hancock’s researches would 
tend to show that these conclusions are generally correct, but 
not entirely so. ‘‘ When the shell is dissolved in acid the free 
border [of the mantle] which projects beyond the marginal fold, 
and which is applied to the extreme edge of the shell, can be 
examined with advantage. The pallial cceca are then completely 
exposed appended to the membrane in various stages of develop- 
ment, and the spaces between them are found studded all over 
with rather large, clear, oval, cell-like spots, which are arranged 
with considerable regularity in rows, so that those in the approxi- 
mate rows alternate. These spots apparently correspond to the 
bases of the prismatic columns of the shell; and if it be allowed 
that they represent spaces in which calcareous granules had 
been accumulated, it is easy to understand how the fibrous or 
columnar structure is formed. <A succession of layers of such 
