THE BRITISH FOSSIL BRACHIOPODA. 297 



both valves are several well-defined muscular impressions, which vary considerably in 

 position and shape in different genera. They occur either in the shape of indentations 

 of greater or lesser depth, or of variously shaped projections. In the Trimerellidce, for 

 example, some of the muscles are attached to a massive or vaulted platform situated 

 in the medio-longitudinal region of the posterior half or umbonal portion of both 

 valves. 1 



The position, number, and functions of the muscles are different in the Tretenterata 

 and the Clistenterata. In various species muscles were attached to the hinge-plate, to the 

 cardinal process, or to septa ; and our knowledge of this subject has been obtained only 

 after much study of the animal itself in both divisions. 



The vascular impressions, differing also in position and shape in different genera and 

 species, are often beautifully displayed on the interior surface of the valves. The 

 ovarian impressions or spaces are also often visible; and indications of the position and shape 

 of the labial appendages, as well as of the mantle itself, have sometimes left imprints on the 

 interior surface of the valves. In addition to these there exists in the interior of the dorsal 

 valve of some genera of Brachiopoda, a variously shaped, thin, calcified, ribbon-shaped 

 lamella or skeleton, which, as stated by Dr. S. P. Woodward, " Supports the brachial 

 membrane, but does not strictly follow the course of the arms. The mode in which the 

 arms are folded is highly characteristic of the Brachiopoda ; the extent to which they are 

 supported by a calcareous skeleton is of less importance and liable to be modified by 

 age." 9 So varied, yet in shape so constant to certain genera and species, are these laminal 

 apophyses, that they have served as one of the chief characters in the determination of both 

 recent and extinct genera. 3 The apophysis, or loop, as it is called, is more or less deve- 

 loped. In some genera it extends to upwards of three fourths of the length of the shell, 

 as in Waldheimia ; but in others it is short, and projects but little beyond the hinge-line. 

 In some genera it is attached only to the hinge-plate, as in Terebratula, Waldheimia, 

 &c. ; in others, to a central longitudinal plate or septum, as in Terebratella, &c. In 

 certain families the apophysis presents the form of two spirally coiled lamellae, which 

 nearly fill the interior of the shell ; the ends of the spirals being directed outwards or 

 towards the cardinal angles, as in S/nrifera, Athyris, &c. ; or horizontally, the apices facing 

 each other towards the centre of the shell, as in Glassia. Again, the spirals are sometimes 

 vertical, their inner sides being pressed together and flattened, with their terminations 

 close together and facing the bottom and centre of the dorsal valve, as in Atrypa. The 

 spiral processes are also connected by a more or less complicated system of lamellae, 

 which have served, in part, as valid grounds for separating into different genera, Spirifera, 



1 Davidson and King, " On the Trimerellidce" ' Quarterly Journal Geol. Soc. of London,' vol. xxx, 

 p. 124, 1874. 



2 « A Manual of the Mollusca,' p. 211, 1856. 



3 In the 'Annals and Magazine of Natural History,' vol. ix, p. 361, May, 1852, I published "A 

 Sketch of a Classification of Recent Brachiopoda based upon Internal Organisation," and gave figures of 

 the loop in different genera. 



