PANAMIC-PACIFIC PELECYPODA 321 



A. Ligament completely internal. 



1. Shell trigonal-ovate in shape, with high, full, centrally placed umbones. 

 Beaks not touching, removed from direct contact with the hinge mar- 

 gin by a narrow or wide space. 



Genus Mulinia 



B. Ligament is partly external, a small segment of it showing normally 

 above the hinge margin behind the beaks. The beaks are adjacent and 

 touch or nearly so at the hinge margin. 



a. The external section of the ligament (tensilium) is small and is at- 

 tached to a short shelly plate formed by an extension of the anterior 

 margin backwards under the beak. The lateral teeth are of unequal 

 size, the posterior lateral tooth small and placed close to the side of 

 the chondrophore. 



2. Shell trigonal in shape, usually high, smooth, thin, the posterior-dorsal 

 areas rather wide and defined by a sharp angle or a winged keel. 



Genus Alactrellor.a 



3. Shell rounded, thin and fragile, its surface marked with deep, con- 

 centric undulations, transmitted equally strong into the interior. Pos- 

 terior-dorsal areas distinct, narrow, set off by a ridge or keel. 



Genus Harvella 



b. The external section of the ligament (tensilium) large or small, at- 

 tached directly to the margin of the shell behind the beaks and separ- 

 ated from the chondrophoral pit by a space or by a small line or 

 ridge. Lateral teeth of nearly the same size and spaced about the same 

 distance from the central chondrophore. 



ba. Lateral teeth smooth. 



4. Shell wall thick or moderately so, the external surface smooth or 

 simply covered with growth lines. Periostracum coarse, usually dark 

 in color. 



Genus Mactra 



bb. Lateral teeth strongly or faintly striated. 



5. Shell ovate to elliptical, either side the longer. Pallial sinus short. Sur- 

 face smooth or with growth line concentrics. Periostracum dark or 

 light-colored. 



Genus Spisula 

 IL Shell obliquely ovate in shape, the anterior-umbonal section strongly 

 convex or inflated, the posterior side depressed. Shell wall usually thin 

 and fragile. Posterior-dorsal slope poorly defined. 



Subfamily Pteropsidinae 



c. Surface marked with deep undulations. 



6. Surface undulations strictly concentric, generally strongest on the 

 umbones, irregular and subobsolete elsewhere. A minute sculpture of 

 fine, radial threads, or vermiculate striae cover the whole disk. 



Genus Raeta 



7. Surface undulations in part strongly oblique to the valve margin and 

 crossed by coarse growth lines and striations. 



Genus Tumbeziconcha 



d. Surface smooth, without deep undulations. 



8. Shell in shape like Anatina, the posterior side depressed and with a 

 small weak ridge near the middle. 



Genus Labiosa. 



Subfamily MACTRENAE 



Shell usually hatchet-shaped, ovate-trigonal, subequilateral, the um- 

 bones submedian, and with small prosogyrate beaks. Hinge as described 



