BRACHIOPODA. 647 



spring from near the hinge of the dorsal valve, while the free end 

 of the loop is folded back upon itself (fig. 492, b). "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 

 Spirifera (fig. 493), Athyris, &c. ; or horizontally, the apices facing 

 each other towards the centre of the shell, as in Giassia. 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 " 

 (Davidson). The spiral processes are also commonly connected by 

 a more or less complicated system of lamellae, which vary in shape 



Fig. 493. — Spirifera hysterica — Carboniferous. The right-hand figure shows the interior of 

 the dorsal valve, with the calcareous spires for the support of the arms, gg, Cardinal angles ; 

 e, Hinge-area ; k, Triangular peduncular foramen. 



and character in different types, and are of great value in the dis- 

 crimination of certain genera. In the Rhynchonellidce, the loop is 

 represented only by two short, slender, curved lamellae; while in 

 some cases the sides of the loop carry minute calcareous spines, 

 showing that even the cirri of the arms were supported by an 

 internal skeleton. In whole families, lastly, as, for example, in the 

 Stropkomenidce and Productidce, the arms are entirely devoid of 

 calcified supports. 



Very commonly, the beaks of the dorsal and ventral valves of 

 the Brachiopoda are separated from one another by a narrower or 

 wider space, which is termed the "hinge-area" (fig. 493, e). The 

 "area" is more or less triangular in form, generally developed on 

 the ventral valve only, but sometimes formed by both valves con- 

 jointly, and commonly transversely striated. In such genera as 

 Spirifera and Cyrtina the hinge-area is of very large size, but it may 

 be very narrow, and in many cases it does not exist at all. Another 

 structure that is commonly present in the Articulated Brachiopods 



