108 CYCLADID.E. 



tumid, nearly 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 



413 . o <^ ./ 



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

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

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

 margin narrow, a little curved ; teeth veiy small ; mar- 

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



give the shell a j)inkish 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. parfinncium, and more olAiijuely truncate 



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



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



characteristic. 



Spheerium occidentale. 



Shell regularly oval, niiuute, pellucid, equipartite, striaj very fmc. 



Cyc/as oralis, Prime, Proc. Bost. Soc. iv. 276 (1852) ; preoccupied. 



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



Sp/urrinm occidentale, Prime, Proc. Acad. xi. 295 (1800), xiii. 407. 



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

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

 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 

 '"^Enili'lA!' "^^'^^ ' height, one fourth of an inch. 

 Found in Vermont, not uncommon. 

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

 Prime. It is said to be remarkable for its completely oval slia])e. 

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

 much raised, as in S. partiimcium. 



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

 but the shells alluded to are not known to me. 



