436 



MOLLUSCA. 



shell is said to be " inequivalve " (fig. 275). On the other 

 hand, a line drawn vertically from the beak of the shell to its 



Fig. 274. Morphology of Brachiopoda. A, Lingula pyramidata (after Morse) : p, Peduncle ; 

 s, Sand-tube, encasing base of peduncle. B, Lingula anatina (after Cuvier) : p, The pe- 

 duncle, c, Waldheimia cranium, with adherent young, attached to a stone (after Davidson) : 

 p, Peduncle ; v, Ventral valve ; d, Dorsal valve. D, Crania Ignabergensis, attached by its 

 ventral valve to a piece of coral (Chalk). 



base (in fig. 275, B, from c to /) would divide it into two 

 equal halves, so that the shell is said to be " equilateral." 

 In the true bivalve Shell-fish (Lamellibrancliiata), on the 



Fig. 275. Rhynchonelfa sulcata. A, Profile view ; B, View of the dorsal surface ; c, View 

 of the base, a, Ventral valve ; ft, Dorsal valve ; /, Base ; c, Beak ; k, Foramen. Lower 

 Cretaceous. 



contrary, the valves of the shell are placed upon the sides of 

 the contained animal, so that they are " right " and " left," 



