LAMELLIBRANCHIATA. 



325 



may generally be determined by the fact that the hinge-ligament 

 is posterior to the two umbones (nates), which have the form of 

 two beak-like prominences projecting over the dorsal edge of the 

 shell (Fig. 250 bis, u), and indicate the point (o^ea^where the 

 development of the valves began. The area (c) is behind the apex, 

 and includes the dorsal posterior side of the shell. The part of the 

 dorsal edge in front of the apex is usually shorter, and contains, 

 at least in the equivalve species, an excavation, the lunula (Fig. 

 250 Us, Z), by means of which the anterior edge can at once be 

 recognized. The apex in some forms (Isocardia, Diceras) is spirally 

 twisted. 



KIS 



M. 



VM 



M.I 



FIG. 250 Ins. Dorsal view of shell of Lucina 

 pennsylvanica. AB anteroposterior axis; 

 CD transverse (right and left) axis ; I 

 lunula ; u umbo ; d ligament ; c area (from 

 Perrier, after Fischer). 



FIG. 251. a, Mactra dliptica in its shell; 

 b, inner surface of left valve of Mactra 

 solida. KIS dorsal siphon (cloacal or 

 exhalent) ; KS ventral siphon (branchial 

 or inhalent) ; P foot ; VM impression of 

 anterior adductor muscle ; HM impres- 

 sion of posterior adductor muscle; Ml 

 pallial line ; Mb pallial indentation. 



The hinge-teeth are placed ventral to the umbo. They generally 

 consist of cardinal teeth placed below the umbo, and of anterior 

 lateral teeth in front of, and posterior lateral behind, the umbo. 

 Hinge-teeth are entirely absent in some genera (e.g., Anodonta, 

 Ostrea). 



The ligament is an elastic structure, and is so arranged that it 

 is either stretched or compressed when the shell is closed by the 

 action of the adductor muscles. In the former case it connects 

 the two valves on the dorsal side of the hinge-line (Fig. 250 bis), 

 and is said to be external; in the latter it is on the ventral side 

 of the hinge, and is called internal. 



