sphj:rium. 107 



Sphseriuni tenue.* 



Ci/clas tenuis, Prime, Bost. Proc. iv. IGl (1851) ; Mon. of Cydadtdce, &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, liglit straw-color ; valves slight, 

 interior straw-color ; hintre-martiin short, narrow, nearly *• temie. 



, "^ \. . . , -, I Enlarged. 



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 inhabit 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. transversum ; 

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

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

 Pisidium (^Prlme). 



Sphserium securis. 



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

 hinder end truncate. 



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



Lewis, Proc. Bost. Soc. v. 122. — Jay, Catal. 4tli cd. 466. 

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



xii. 34. 

 Ci/diis cardissa,VRiyiK, Vruc. Bost. Soc. iv. 160, 277. 

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



(1858). 

 Ci/das crucea, Lewis, Proc. Bost. Soc. v. 25 (1854), vi. 2. 



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

 more elevated posteriorly and obliquely truncate, beaks elevated and 



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

 filled up from Mr. Prime's Monograph of Ci/dadid(e, pubHshed 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. 



