T. Davidson — Wliat is a Brachiopod ? 
149 
and middle divisions, which constitute the entire thickness of tlie 
valve, are calcareous, with a prismatic or fibrous structure, while 
the outer division consists of a very thin membrane (Pl.YII. Fig. 29). 
The innermost and intermediate divisions are in some families 
traversed by minute tubulär canals whicli pass from one surface to 
the other, for the most part in a vertical direction, and at tolerably 
regulär intervals; but just before terminating, near the outer surface 
of the epidermis, their orifices suddenly become dilated ; the lower 
half of the canals being often considerably smaller in diameter than 
the upper half. The canals are occupied by coecal processes pro- 
ceeding from the mantle, or the fleshy covering of the animal. Their 
functional nature is according to Dr. Carpenter branchial or sub- 
servient to respiration ; but, as observed by Prof. King, the outer 
epidermis, which covers their expanded terminations, would seem- 
ingly prevent any communication between the surrounding sea- water 
and the mantle ; so that it may be questioned whether they are at 
all connected with the respiratory function. In certain genera, such 
as Bhynchonella, there are no canals ; the shell being found to con- 
sist of flattened prisms of considerable length, which lie parallel to 
one another with great regularity, and at a very acute angle to the 
surface of the valves. The shell substance of Lingula Prof. King 
believes to be also almost entirely composed of Keratode, with a 
subordinate portion of phosphatic calcareous matter. 
The shell structure of Lingula and Discina has also been attentively 
studied by Dr. Gratiolet in his excellent memoir “ Etudes auatomiques 
sur la Lingula anatina,” 1860. Therein he mentions, that “the 
structure of the valves in the Lingulidce appears at first siglit to 
differ much from that of the Terehratulidce, but a more attentive 
examination reveals analogies at first unsuspected. Two distiuct 
elements exist in the shell of Lingula — the one corneous, and the 
other testaceous. They occur in thick layers altemating from the 
convex to the concave surface of the valves, coramencing with a 
superficial corneous one. These layers are not, however, of equal 
thickness throughout. On the convex side the thickness of the horny 
layers is greater towards the outer surface of the shell, while on 
the visceral, or inner surface, the testaceous elements predominate. 
They are of especial thickness at the level of the posterior angle of 
the rhomb. The thick testaceous layers are separated by thin 
corneous ones which thin out at certain points. This arrangement 
gives an amount of opaqueness to the central portions of the valves, 
while the outer edges, where the horny substance predominates, are 
semi-transparent. The structure of the testaceous layers simulates 
that of the Terehratulidce ; they are crossed by numerous microscopic 
canals which are traversed by extremely delicate striae, recalling the 
chains formed by the prismatic elements of the shell in Terehratulidce. 
This description applies equally to Discina, but in these last the 
calcareous portion predominates greatly over the horny one.” 
Soft Parts of the Animal. — I must now allude to some of the soft 
parts of the animal, but it would require much more space than can 
be bestowed in this short article to do anything like justice to so 
