SPHvEUIUM. 223 



vulde iuflato ; latere postico magis producto, subattenuato, anguste 

 truncate. 



Hub. Tenasserim. 



The dimensions of a specimen in the British Museum are: 



Long. 14, lat. 17, diam. 11 mm. 



A pale, uniform greenish-yellow shell ornamented with distant, 

 concentric riblets ; somewhat elongately ovate and considerably 

 inflated, especially remarkable for its thin texture. 



Subfamily SPH^RIIJSLE. 



Shell small, ovate or trigonal, lateral and cardinal teeth more 

 or less developed, ligament external ; palliul line entire. 

 Distribution. World-wide. 



Genus SPHJERIUM. 



Sphcerium, Scopoli, 1777 ; Scopoli, Introduct. p. 397. 



Chama, d'Argenville, Hist. Nat. Lithol. Coiichyl. no. 9, 1742, 



pp. 368, 374, pi. 31. 



Teltina, Linnaeus ; Linn. Syst. Nat. 10th ed. i, 1758, p. 678. 

 Cardium, DaCosla, Brit. Conch. 1778, p. 173, pi. 13, tig. '2, 

 Cydas, Bruguiere, Encycl. M6th. 1792, pi. 301. 

 , Humphrey, Mus. Calonn. 1797, p. 59. 



Amesoda, llatinesque, Ann. Gen. Sci. Phys. et Nat. v, 1820, p. 310. 

 Cycladitex, Kriiger, Gesch. Urwelt. ii, 1823, p. 469. 

 Pisidium, Verany, Descris. Geneva, ii, 1846. 



TYPE, Sphcerium corneum, Linn. ; Europe. 



Range. World-wide. 



Shell thin, ovate, convex, subequilateral, covered with a fine 

 periostracuin, smooth or concentrically striate ; umboues lightly 

 anteriorly inclined ; anterior side rather shorter than the 

 posterior ; margins of the valves simple ; cardinal margins 

 thin; cardinal teeth small, that in the right valve often bifid, 

 fitting between two oblique teeth in the left ; both anterior and 

 posterior lateral teeth compressed, divergent, somewhat pro- 

 jecting, lameUiform double in the right valve, single in the left ; 

 ligament external, weak, scarcely apparent ; pallial line entire. 



Animal having the edge of the mantle smooth ; foot tongue, 

 capable of considerable extension ; siphons moderately long, 

 united at their base, separated at the end ; the branchial siphon 

 somewhat the longer and broader ; siphonal orifices simple ; 

 palpes triangular, lanceolate; gills unequal, the outer the shorter 

 and fringed. 



