﻿T. Davidson — What is a Brachiopod ? 149 



and middle divisions, wliicli constitute the entire thickness of the 

 valve, are calcareous, with a prismatic or fibrous structure, while 

 the outer division consists of a very thin membrane (PI. VII. Fig. 29). 

 The innermost and intermediate divisions are in some families 

 traversed by minute tubular canals which pass from one surface to 

 the other, for the most part in a vertical direction, and at tolerably 

 regular 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 ccecal 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 Ehynclionella, 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 anatomiques 

 sur la Lingula anatina," 1860. Therein he mentions, that " the 

 structure of the valves in the Lingulidce appears at first sight to 

 diifer much from that of the Terebratulidop., but a more attentive 

 examination reveals analogies at first unsuspected. Two distinct 

 elements exist in the shell of Lingula — the one corneous, and the 

 other testaceous. They occur in thick layers alternating from the 

 convex to the concave surface of the valves, commencing 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 Terebratulidoe ; they are crossed by numerous microscopic 

 canals which are traversed by extremely delicate stri^, recalling the 

 chains formed by the prismatic elements of the shell in Terebratulidai. 

 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 



