//. ACEPHALA. 



>9 



Usually there occur an anterior and a posterior adductor equally 

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

 mentary (Ileteromyaria) or entirely disappears (Monomyaria). 



Fio. 347. 



Fig. 348. 

 Fig. 347.— Left valve of Criis.sati:U(' i^lunihca, inner and outer surfaces. (From Zittel.) 



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

 Fig. 34K.— Right valve of Mactra .itultnrum, with paUial sinus. (From Ludwig-Leunis.) 



Letter.s for both figures: a', anterior; a", posterior adductor scar; c, hinge; (, 



internal ligamental groove; ?/(, paUial line ;.■>, paUial sinus. 



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

 ligament opens the valves. 



Tlie heterodout binge is the typical form (fig. 348); eacli valve bears a 

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

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

 lateral teeth,^ often jiroduced into ridges. The ligament lies behiud 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- 

 dout. Then there are Acophala of apparently primitive character which 

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

 in a series symmetrical to the umljo (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 jjlace most rapidly at the edge 

 of the mantle, Isoth arc closely uinted, the union hoing strength- 

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

 appearance from the rest, this part being marked <>tf by a pallial 

 line parallel to the margin (fig. 347). In many sjjecies, the Siuu- 



