46 AMERICAN' CORBICULAD-E. 



a somewhat rlionibiform appearance ; cardinal teeth strong, assuming ths 

 shape of the letter V reversed ; lateral teeth very much elongated. 



The young shell is more compressed than the adult ; it is usually light 

 yellow. 



Long. 0.50 ; Lat. 0.43 ; Diam. 0.31 inches. 



Hub. North America, in the States of New England, New York, New 

 Jersey, Wisconsin, Pennsylvania, Ohio, Michigan, South Carolina, Georgia, 

 Mississippi, Alabama, and Arkansas. (Cabinets of the Academy of Natural 

 Sciences of Philadelphia, Smithsonian Institution, Boston Society of Natu- 

 ral History, Garden of Plants at Paris, Agassiz, Jay and Prime.) 



Cijclus partumeia, Say, Acad. Nat. Sci. Phila. Jour. II, 1822, 380. 

 Ci/clas cornea, var. 2, Lamakck, An. s. Vert. V, 1818, 558. 

 Cyclas orbicularia, Barratt, American Jl. XLVIII, 1845, 276. 

 Cyclas mirabiiis, Prime, Best. Proc. IV, 1851, 157. 

 Cyclas ccerulea, Prime, loc. sub. cit. IV, 1851, Kil. 

 Cyclas eburnea, Anthony, loc. sub. cit., IV, 1852, 279. 



This species varies much according to the localities where it is 

 fouud, whicli accounts in part for the number of names it has re- 

 ceived. G. orhicularia, of which I have authentic specimens 

 from Mr. Barrat, is a genuine S. partumcium, without even any- 

 local modifications of shape. G. mirabiUs, from Georgia, is a 

 small form of this species, and G. ccerulea differs from the type 

 in being a little less inflated. G. eburnea, from Arkansas, varies 

 from the northern S. jMrtumeium in being more compressed and 

 a little more elevated. I do not think, however, taking the differ- 

 ence of localities into consideration, that these are characters 

 sufficient to warrant retaining G. eburnea as a distinct species. 

 I had an opportunity, while in Paris, to assure myself that the 

 variety No. 2 of G. cornea was a true S. jyaftumeiuvi. 



This species is not only very widely distributed, but where it is 

 found, it occurs in large numbers. The only one of our northern 

 species to which it bears much resemblance is S. truncahim, and 

 that is only in general outline; the S. partumxeiumiB much more 

 inflated and transversely more broad. 



20. Spliaeriuin iayaiiiini, Prime. — Animal not observed. 



Shell rhombic, nearly equilateral, moderately convex, thin, fragile, 

 somewhat translucent, drawn up to an angle towards the hinge-margin ; 

 anterior and posterior margins very abrupt, inferior very slightly curved ; 

 beaks central, calyculate, approximate at apex ; hinge-margin consider- 

 ably shorter than the basal margin, slightly curved ; cardinal teeth 



