296 MOLLUSCOIDEA SUB-KINGDOM V 
projections, one on either side of the delthyrium, called the hinge-tecth (Fig. 
498, 6), which fit into the so-called dental sockets of the 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 

Fia, 494. 
Different forms of brachidia. A, Rhynchonella ; fleshy arms supported by two simply curved crura. B, The- 
cospira ; inwardly coiled double spires, or spiralia. C, Nucleospira ; and D, Cyrtina ; outwardly coiled spiralia. 
E-H, Loops of Brachiopods. £E, Centronella ; F, Dielasma; G, Terebratella; 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, 4). 
Some forms are also provided with lateral septa (Thecidiidae). ws 
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 Lhynchonellacea and 
Pentameracea, where it consists of two short, or only moderately long, curved 
processes called the crura when discrete, and cruraliwm when the plates are 
united. The crura are attached to the hinge-plates. The cruralium is 
