//. ACEPHALA. 



359 



Usually there occur an anterior and a posterior adductor equally 

 well developed (Dimyaria); less frequently the anterior is rudi- 

 mentary (Heteromyaria) or entirely disappears (Monomyaria). 



FIG. 348. 

 FIG. 347. Left valve of Crassatella plumhea, inner and outer surfaces. (From Zittel.) 



The outer surface showing lines of growth ; no pallial sinus. 

 FIG. 348. Right valve of Mactra gtuitorwm, with pallial sinus. (From Lud wig-Leunis.) 



Letters for both figures: a', anterior; a", posterior adductor scar; e, hinge; I, 



internal ligamental groove ; m, pallial line ; s, pallial sinus. 



When the muscles are relaxed (as always occurs at death) the elastic 

 ligament opens the valves. 



The heterodont hinge is the typical form (fig. 348); each valve bears a 

 group of teeth near the umbo, those of the left alternating with those of 

 the right. Besides these ' cardinal teeth ' there are in front and behind 

 'lateral teeth? often produced into ridges. The ligament lies behind the 

 hinge and is usually visible from the outside (external ligament), but is 

 occasionally transferred to the interior (internal ligament, fig. 347). The so- 

 called schizodont and desmodont hinges are modifications of the hetero- 

 dont. Then there are Acephala of apparently primitive character which 

 either lack the hinge (dysodont), or have one composed of numerous teeth 

 in a series symmetrical to the umbo (taxodont), or of two strong teeth like- 

 wise symmetrical to the umbo (isodont). In these cases the ligament is 

 developed in front of as well as behind the umbo, and may be either 

 external or internal. 



Since the secretion of shell takes place most rapidly at the edge 

 of the mantle, both are closely united, the union being strength- 

 ened by small muscles. So the edge of the shell has a different 

 appearance from the rest, this part being marked off by a pallial 

 line parallel to the margin (fig. 347). In many species, the Sinu- 



