PELECYPODA 587 



2. PisiDlUM Pfeiffer. Shell minute, round or ovalj 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: 



a^ Shell triangular P. compbessum 



a. Shell round or oval. 



hi Shell without ventral yellow band P. abditum 



62 Shell with darker yellow ventral band P. vabiabile 



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 pi^i^um 

 umbo veiy much elevated, 4.5 mm. long and high, and 3.1 makerT 



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 equivalve, 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: 

 Ci Pallial sinus very large; siphons separate. 



61 Ligament external. 



Ci Inner margin of shell not crenulated. 1. Tellinidae 



C2 Inner margin of shell crenulated 3. Donacidae 



62 Ligament partly internal, inserted in a pit in the shell 2. Semelidae 



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. 



