SPHJIRIUM. 



107 



Sphserium tenue.* 



Cyclas tenuis, Prime, Bost. Proc. iv. 101 (1851) ; Mon. of Cijcladida, &c. 



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

 equilateral ; anterior and l)asal marg-ins rounded, posterior 

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

 calyculate ; striations very fine and regular, hardly per- 

 ceptilde ; epidermis glossy, light straw-color ; valves slight, 

 interior straw-color ; hinge-margin short, narrow, nearly 

 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 ol> 

 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 asi<le 

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

 mature shell. In outline it seems to be allied to *S'. trans ver sum ; 

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

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

 Pisidiam {Prime). 



SpliEerium securis. 



Shell .small, rliomboidal, 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 cd. 466. 

 Sphcerinm securis, Adams, Gen. ii. 450 (1858). — Prime, Proc. Acad. Sc. xi. 298 (1800), 



xii. 34. 

 Cyclas carrf/ssff, Prime, Proc. Bost. Soc. iv. 160, 277. 

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



(1858). 

 Cijdas crucea, Lewis, Proc. Bost. Soc. v. 25 (1854), vi. 2. 



Shell small, trapezoidal, slightly obli(]ue, acutely rounded in front, 

 more elevated posteriorly and obliquely truncate, l»eaks elevated and 



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

 filled np from Mr. Prime's Monograph of C'l/cladidcr., pnblished liy the Smithsonian Insti- 

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

 work, and Mv. Prime has also furnislied some. — W. G. B. 



