SPH.ERIUM. 107 



Sphserium tenue.* 



Cyclas tenuis, Prime, Bost. Proc. iv. 161 (1851) ; Mon. of Cydadida, &c. 



Shell small, transversely oblong, pellucid, moderately full, sub- 

 equilateral ; anterior and basal margins rounded, posterior 

 margin sub-abrupt ; beaks nearly central, not prominent, ^'^' 

 calyculate ; striations very fine and regular, hardly per- 

 ceptible ; epidermis glossy, light straw-color ; valves slight, ^_ 

 interior straw-color ; hinge-margin short, narrow, nearly I^^^^J?^'/^ 

 straight ; cardinal teeth very diminutive, lateral teeth 

 small, elongated. Length, eighteen hundredths of an inch ; breadth, 

 twelve hundredths of an inch ; width, six hundredths of an inch. 



In the Androscoggin, Maine, and in the Upper Mackenzie, Brit- 

 ish America. 



This species, the smallest one known to inhaljit the United States, 

 was discovered some years since by Mr. Girard, from whom I ob- 

 tained my specimens. It may possibly be the young of some spe- 

 cies, but if so, it would be very difficult to say which ; setting aside 

 its diminutive size, it appears to have all the characteristics of a 

 mature shell. In outline it seems to be allied to S. iransversum ; 

 it is, however, more inflated, less elongated, and its margins are 

 more rounded. At first sight it might readily be mistaken for a 

 Pisidium ( Pr ime) . 



Sphserium securis. 



Shell small, rhomboidal, disks tumid, pinched at each end, beaks elevated, 

 hinder end truncate. 



Cyclas securis. Prime, Proc. Bost. Soc. iv. 160, 276 (1852) ; Ann. Lye. v. 218, pi. 6. — 



Lewis, Proc. Bost. Soc. v. 122. — Jay, Catal. 4th ed. 466. 

 SphcBrium securis, Adams, Gen. ii. 450 (1858). — Prime, Proc. Acad. Sc. xi. 298 (1860), 



xii. 34. 

 Cyclas cardissa,V RIME, Vtoc. Bost. Soc. iv. 160, 277. 

 Sphceriuin cardissum, Prime, Proc. Acad. Se. xi. 289 (1860). — Adams, Gen. ii. 450 



(1858). 

 Cyclas crocea, Lewis, Proc. Bost. Soc. v. 25 (1854), vi. 2. 



Shell small, trapezoidal, slightly olilique, acutely rounded in front, 

 more elevated posteriorly and obliquely truncate, beaks elevated and 



* Tliis species was represented in Dr. Gould's MSS. by a blank sheet, which I have 

 filled lip from Mr. Prime's Monop;raph of Cycladidce, published by the Smithsonian Insti- 

 tution. Professor Henry has kindly allowed me to use the wood-cuts prepared for that 

 work, and Mr. Prime has also furnished some. — W. G. B. 



