﻿148 T. Davidson — What is a BracMopod'^ 



mainly of the brachial or labial appendages ; and so varied, yet con- 

 stant in shape to certain species is this laminal apophysis that it has 

 served as one of the chief characters in the creation of both recent 

 and extinct genera (PL IX. Figs. 2, 7, 9). The apophysis, or loop, 

 is more or less developed, extending in some genera to upwards of 

 three-fourths of the length of the shell, but in others it is so short 

 and rounded as to project only a little beyond the hinge. In some 

 genera it is attached only to the hinge-plate, as in Terebratula,Wald- 

 heimia (PI. IX. Fig. 2), in others to a central longitudinal plate or 

 septum' (PI. IX. Fig. 9). In certain families the apophysis presents 

 the form of two spiral processes which nearly fill the interior of the 

 shell, the ends of the spires being either directed outwards towards 

 the cardinal angles (Spirifer, PL VIII. Fig. 6), or placed horizontally 

 with their apices directed inwards and towards the centre of the 

 concave surface of the same valve, which they almost fill ; the inner 

 sides of the spires are pressed together and flattened with their ter- 

 minations close to each other near the centre of the bottom of the 

 shell {Atrypa, PL VIII. Fig. 7). In the BhynchonelUdce again it 

 assumes the shape of two short, slender, curved lamina3 (PL IX. 

 Fig. 7) ; while in many genera and even families, Productidce, etc., 

 there exists no calcified support for the labial appendages. 



The muscles, pallial sinuses, and ovaries, generally leave such in- 

 dentations on the internal surface of the valves, in both recent and 

 fossil genera and species, that they materially assist in determining 

 the characters of extinct forms ; and in addition, the brachial or 

 labial appendages often leave indications of their presence and cha- 

 racter, which is also evinced by the shape of the calcified skeleton 

 which supported them to a greater or lesser extent. 



Shell Structure. — The structure of the shell has been shown by 

 Dr. Carpenter, Pi'of. King, Dr. Gratiolet and others to be generally 

 distinct from that of the Lamellibranchiata or Gasteropoda. Dr. 

 Carpenter, who has described the shell structure with infinite care 

 and minuteness, informs us that in the shell of the Brachiopoda, 

 there is not that distinction between outer and inner layers, either in 

 structure or mode of gi'owth, which prevails among the ordinary 

 bivalves ; and that it seems obvious, both from the nature and form 

 of the shell substance, the mode in which it is extended, that the 

 whole thickness of the Brachiopod shell corresponds with the outer 

 layer only of the Lamellibranchiata. He adds that he has, however, 

 occasionally met with a second layer in recent Terebratulce, within 

 the earlier formed portion of the shell, but confined only to a part 

 of the surface, instead of extending beyond it. In some families, 

 according to Prof. King, it consists of three divisions ; the innermost 



1 In a very interesting paper on the development of the loop in Wald. cranium 

 and W. septigera entitled " Bidrag til Vestlandets Molluskfauna," published in the 

 *' Saerskilt Aftryk af Videnskabsselskabets Forhandlinger " for 1875, Mr. Herman 

 Friele has started the hypothesis that the loop becomes modiiied with age, and he 

 indicates a most remarkable development of a simple loop out of a compoimd one, 

 but before this important question can be definitely settled a complete connecting 

 series of the different ages of a same species will have to be examined, which has not 

 been hitherto done to my entire satisfaction. 



