360 



MOLLUSCOIDEA 



PHYLUM V 



dorsal valve in the Protremata. It is particularly well developed in the families 

 Clitambonitidae and Strojjhomenidae (Fig. 585, B). The chilidinm is not to be homo- 

 logised with the deltidium, since it never appears earlier than the adolescent stage, 

 and is aj)parently a secretion of the dorsal mantle lol^e. Both its origin and phyletic 

 significance are therefore very different from those of the deltidium. 



The listrium is a plate closing the progressive track of the pedicle opening or 

 pedicle cleft, in some Neotremata, posterior to the apex of the ventral valve. 



Internal Characters of the Shell : Articulation. — The two valves are 

 held in apposition either by muscles only (Atremata and Neotremata), or they 

 are united by articulation (Protremata and Telotremata). In the latter case 

 there are to be seen in the ventral valve a pair of cuneate or tooth-shaped 

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

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

 lation is also aided by the cardinal jyrocess, 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. 535, A), 

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



Fi(i. 531. 

 Different forais of brauhidia. A, Hcmithiiriti ; flesliy arms supported by two simply curved crura. B, Theco- 

 spira- inwardly coiled double spires, or spiralia. C, Nucleospira ; and D, Cyrtina; outwardly coiled spiralia. 

 h-H, J^oops ot Brachiopods. E, Centronella ; F, Dielasma ; G, TerebraUlla ; H, Megatltyris. 



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 

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

 valve, and may sometimes extend as far as the anterior margin (Fig. 535, A). 

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



