108 CYCLADID.E. 



tumid, ncai'ly central, dorsal aspect broad, acutely rhomboidal, pre- 

 senting a broad, deep, and long depression each side of the beaks ; 

 side aspect tumid down the disk, somewhat pinched and prolonged 

 at each end so as to give the appearance of a ridge from the apex 

 to each ventral end ; ventral margin slightly curved and 

 the adjacent part of the disk wedge-shaped ; end views 

 heart-shaped ; surface with fine concentric ridges, shin- 

 ing, horn-color with occasional yellowish zones. Hinge- 

 s secuns. margin narrow, a little curved ; teeth verv small : mar- 

 ginal teeth slight, elongated. Animal pinkish, so as to 

 give the shell a pinkish hue. Diameter about one third of an inch ; 

 breadth, one fourth of an inch. 



Inhabits Fresh Pond, Cambridge, and Salem ; also Vermont, New 

 York, and Pennsylvania. 



Less lenticular than S. partumeium, and more obliquely truncate 

 than S. truncatuni; it is smaller, thicker, more tumid, and more 

 shining than either, and the peculiar pinching at the ends is quite 

 characteristic. 



Sphserium occidentale. 



Shell regularly oval, minute, pellucid, equipartite, stria3 very fine. 



Cyclas ovalis, Prime, Proc. Bost. Soc. iv. 276 (1852) ; preoccupied. 



Sphcerium ovale, Adams, Gen. ii. 450 (1858). 



Sphcerium occidentale, Prime, Proc. Acad. xi. 295 (1860), xili. 407. 



Shell small, pellucid, fragile, transparent, equilateral, somewhat 

 elongated, not much inflated, outline of the valves oval ; beaks 

 small, rounded, not prominent ; lines of growth fine 

 '"■ and regular ; color in some a light yellow, in others 



greenish yellow ; hinge-margin very gently rounded ; 

 teeth very diminutive, laterals more distinct. Length, 

 one third of an inch ; breadth, three sixteenths of an 

 '^■EniMged"^^' "^^^^ ' I'eight, one fourth of an inch. 

 Found in Vermont, not uncommon. 

 I have not seen this shell, and derive the description from Mr. 

 Prime. It is said to be remarkable for its completely oval shape. 

 It is smaller, the margins more rounded, and the beaks not so 

 much raised, as in S. partumeium. 



There are some other species indicated as found within our limits, 

 but the shells alluded to arc not known to me. 



