INVERTEBRATE PALAEONTOLOGY. 525 



Genus SPHJERIUM, Scopoli. 

 Pyvw -Spharium, Scopoli (1777), Introd. Hist. Nat., 397.— Gray (1847), Zoiil. Proceed., 184.— II. and A. 

 Adams I l-.".7). Genera of Recent Moll., II. 449.— Prime (1860), Synon. Family Cydades, 

 Proceed. Acad. Nat. Sri. Pbilad., XII, 2: and Monogr. Species Sphwrium, 3; also 

 Mouogr. Am. Coroiculidce, 32.— Meek and Hay den (1860), Proceed. Acad. Nat. Sci. 

 Pbilad., XII, 175. — Meek (1866), in Conrad's Smithsonian Check-List N. Am. Eocene 

 Moll., 6.— Gonld (1870), Invertebrata Mass. (Binney ed.), 101. 



Cydas (part), Brugniere (1792), Encyc. M< < th., pi. 301 and 302.— Lamarck (1799), Prodr., 84; 

 also (1801) Syst. An. sans Vertebr., 123; and (1818) Hist. Nat., V, 557.— Blainv. 

 (1825), Mala,-., 551.— Fleming (18-28), Brit. An., 409 and 452.— Swainson (1840), Malac, 

 369.— Gould fl841), Invertebrata Mass.. 72.— G. B. Sowerby, jr. (1842), Conch. Man. 

 (2d ed.), 130 ; and of numerous others (not of Klein, 1753). 



Xus, Humphrey (IT',17). Mus. Col. 



Museulnm, Link (1807), Rost. Samml., III. 152 (not Gray, 1851). 



Cornea, Miihlf. (1811), Entwurf, 56. 



Comeocydas (FcSruss.), Blainv. (1818), Diet. Sci. Nat., XII, 278. 



Amesoda, Raf. (1820), Mon. Biv, 319. 



Cydadiles, Kriig. (1823), Vrw., 11, 469. 



Etym. — a<paipiov, a little ball. 

 Type. — Tellina cornea, Linna;us. 



Shell thin, oval-cordate, nearly equilateral, more or less gibbous ; 

 smooth or concentrically striated ; hinge usually with two small, moderately 

 diverging cardinal teeth in each valve, or sometimes one in one valve and 

 two in the other, or one in each valve, rarely with both nearly obsolete ; 

 lateral teeth more or less elongated, smooth, double in the right valve and 

 single in the left; ligament narrow, and placed on the longer side of the shell ; 

 muscular and pallial impressions faintly marked, the latter being nearly 

 or quite simple. 



The species of this genus inhabit fresh-water lakes, ponds, and streams 

 of cold and temperate latitudes, especially of North America and Europe. 

 Their habits are very similar to those of Pisidium, which they nearly resemble 

 in form and general appearance. They differ, however, in having the beaks 

 and cardinal teeth more nearly central, and the ligament on the longer side. 

 The animal of this genus also differs from that of Pisidium in having its 

 siphons longer and disunited at their free ends. 



The genus Sphcerium dates back to the Wealden epoch. It also occurs 

 in this country in beds that may belong to the latest members of the Creta- 

 ceous, as well as in others of the Tertiary system. According to Prime, 

 about one hundred species are known to exist at the present time. 



The following-described species are referred to this genus rather than 

 to Pisidium from general form ; none of the specimens being in a condition 

 to give any clear information in regard to the characters of the hinge, beyond 

 the tact of the existence of moderately-elongated, smooth, lateral teeth in a 

 few of the specimens. 



