PELECTPODA 587 



2. PISIDIUM Pfeiffer. Shell minute, round or oval; greenish or 

 yellowish umbo a little back of the middle, and directed backwards; 2 

 cardinals in the left and 1 in the right valve; laterals lamelliform, 1 in 

 the right and 2 in the left valve ; anal siphon alone present ; foot tongue- 

 shaped, capable of great extension: 60 species; cosmopolitan; about 20 

 in America. 



Key to the species of Pisidium here described: 



G! Shell triangular P. COMPBESSUM 



a a Shell round or oval. 



fe x Shell without ventral yellow band P. ABDITUM 



6 2 Shell with darker yellow ventral band P. VABIABILE 



a a Pallial sinus very large ; siphons separate. 



P. compressum Prime. Shell solid, triangular, inflated, very oblique ; 

 umbo elevated, with a wing-like appendage on the summit ; dorsal margin 

 acutely arched; length and height 4.5 mm.; width 3 mm.; color yellow 

 or brownish: entire country; often common. 



P. abditum Haldeman (Fig. 924). Shell oval, solid, 4.25 mm. long, 

 3 mm. high, and 2.5 mm. wide; surface smooth, shining, 

 bright yellowish in color; 1 arched cardinal in the right 

 and 2 in the left valve ; laterals strong and curved : entire 

 country; the commonest species. 



P. variabile Prime. Shell round, solid, inflated, with 

 umbo very much elevated, 4.5 mm. long and high, and 3.1 ?Bak*erf 

 mm. wide; surface smooth with heavy lines of growth, 

 light yellow or greenish in color, with a yellow ventral zone: entire 

 country north of Virginia; common. 



SUBORDER 2. TELLINACEA. 



Shell equi valve, elongate or more or less triangular; pallial sinus 

 very large, extending forwards sometimes almost to the anterior adductor 

 muscle; mantle edges usually open below; siphons very long, and usually 

 separate; foot and palps large; 2 adductor muscles: 7 families; marine. 



Key to the families of Tellinacea here described: 

 Oj Pallial sinus very large ; siphons separate. 

 & x Ligament external. 



Ci Inner margin of shell not crenulated 1. TELLINIDAE 



c 2 Inner margin of shell crenulated 3. DONACIDAE 



& 2 Ligament partly internal, inserted in a pit in the shell 2. SEMELIDAE 



c a Pallial sinus not so large ; siphons fused 4. MACTBIDAE 



FAMILY 1. TELLINIDAE. 



Shell compressed, equivalve, with 1 or 2 cardinal teeth, and usually 

 1 lateral in each valve \ pallial sinus very large ; ligament external, promi- 

 nent ; mantle widely open below and in front ; siphons very long, slender, 

 and separated : over 600 species, in all seas, chiefly littoral. 



