296 



MOLLUSCOIDEA 



SUB-KINGDOM V 



projections, one on either side of the delthyrium, called the hinge-teeth (Fig. 

 498, ), which fit into the so-called dental sockets of tjie dorsal valve. Articu- 

 lation is also aided by the cardinal process, which is a more or less well- 

 developed apophysis of the dorsal valve, and is received between the teeth of 

 the ventral valve. By the contraction of the muscles attached to the 

 cardinal process, the valves are opened along the anterior and lateral 

 margins ; but when shut, the test entirely encloses the soft parts of the 

 animal. 



The dental sockets are bounded on the inside by hinge-plates (Fig. 498, A), 

 which are often supported by vertical or inclined septal plates extending to 

 the bottom of the shell. The teeth of the ventral valve are sometimes 

 supported by lamellae known as the dental plates. In addition to the dental 

 plates, which frequently attain considerable size, there may be a median 



FIG. 494. 



Different forms of brachidia. A, Rhynchonella ; fleshy arms supported by two simply curved crura. B, Thc- 

 cospira ; inwardly coiled double spires, or spiralia. C, Nudeospira ; and D, Cyrtina ; outwardly coiled spiralia. 

 E-H, Loops of Brachiopods. E, Centronellu; F, Dielnsma; G, Terebmtella ; H, Megathyris. 



septum of variable proportions. This may begin beneath the beak of the 

 valve, and may sometimes extend as far as the anterior margin (Fig. 498, A). 

 Some forms are also provided with lateral septa (Thecidiidae). 



Brachial supports. Of special systematic importance are the brachidia, or 

 internal skeleton of the fleshy arms (Fig. 494), which occur in the Spiriferacea 

 and Terebratulacea. The brachidia are, as a rule, united basally with the 

 crura, and are extremely variable in form. They usually pass through a 

 more or less complex series of metamorphoses during the growth of the 

 individual, and do not attain their complete development until the animal has 

 reached maturity. 



The simplest form of brachial supports is found in the PJiynchonellacea and 

 Penlameracea, where it consists of two short, or only moderately long, curved 

 processes called the crura when discrete, and cruralium when the plates are 

 united. The crura are attached to the hinge-plates. The cruralium is 



