Fam. 21.] NATURAL HISTORY SOCIETY. 103 



pallial sinus deep; surface with fine, concentric wrinkles; a slight, 

 wave-like, gonial ridge. White, covered with whitish or dusky 

 epidermis. Very variable in size, solidity and color. In muddy 

 bays, they grow to a large size, are thick and bluish, and have a 

 thick epidermis ; in clear, sandy localities, they are delicate, thin, 

 pure white to pink. L. 23 ; H. 18 ; W. 9 ; U. i— Whole coast. 

 Abundant. 



3. Macoma subidosa, Speng. Shell subovate, thin, brittle, 

 slightly gaping ; anterior end longer and well rounded ; posterior 

 end wedge-formed, with a somewhat truncate end ; gonial ridge 

 a roughened wave ; pallial sinus reaching nearly to the anterior 

 muscular impression; surface undulated by the lines of growth. 

 Epidermis thin, dirty white to dusky ; interior, bluish white. 

 L. 23; H. 15; W. 6; U. |; An. 140°.— Conn, and north. 

 3. Gastranella, Verrill. 



Shell oblong, more or less irregular, and sometimes with the 

 ventral margin inflexed ; pallial sinus large ; ligament external, 

 elongated ; right valve with two small cardinal teeth, the pos- 

 terior one thin and oblique ; left valve also with two, the posterior 

 one stout, bilobed ; no distinct lateral teeth. Animal with long, 

 slender, fringed, separated siphon tubes; mantle well open in 

 front ; foot strap shaped. 



1. Gastranella tumida, Verrill. Shell elongated-oval or oblong 

 with rounded ends, compressed posteriorly ; umbones rounded, 

 somewhat prominent, incurved but not approximate, directed 

 forward; no distinct lunule, but deeply concave in front of um- 

 bones; ventral margin variable, slightly convex, straight or 

 sometimes even concave; posterior end usually rounded some- 

 times decidedly prolonged ; surface marked with fine, irregular 

 lines of growth or striae, slightly iridescent. White with purple 

 umbones. L. 4 ; H. 2^; W. 1^; U. J. 



Family 21. SEMELID/E- 



Shell thin, subequivalve, gaping and usually twisted at the 

 posterior end ; with both a short external ligament and an inter- 

 nal cartilage. Siphons long, separated. 



* Elongated shells 1. Cumingia. 



* Length and height nearly equal 2. Semele. 



1. Cumingia, G. B, Sowb. 

 Shell oval, equivalve, rounded in front, somewhat rostrated 



