102 MAZATLAN BIVALVES 



centric ridges slightly decussated. They are evidently young, 

 extremely thin, and shaped like the fossil F. lamellosa. 

 Long. '06, lat. '08, alt. '04. 

 Hob. Mazatlan ; off Spondylus calcifer, 2 sp. ; L'pool Col. 



GENUS DIPLODONTA, Bronn. 



Diplodonta, Bronn 1831, Italiens Tertiargeb. p. 9. 



Mysia, Gray, Proc. Zool. Soc. 1847, p. 195, (non Mysia Leach,) 



teste Phil, in Handb. der Conch, fy Mai. p. 342. 

 (rlocomene, Leach (a see-undo, manu,) Br. Moll. 313. 



For a full account of the remarkable peculiarities of the 

 animal of D, rotundata, v. Clark Moll. Test. Mar. Br. p. 82. 



150. DIPLODONTA SEMIASPERA, ? Phil. 



Quoted by Philippi, in Abbild. Conch. Tellina, p. 25, under 



Tellina pisiformis, Aug. 1846. 

 ? = Lucina orbella ; Gould, Proc. Bost. Soc. N. H. Nov. 1851, 



vol. iv. p. 90 : Cal. $ Mex. Shells, p. 22, pi. 15, f. 3. 

 ?=Lucina semireticulata, B. M. Cat. I)' Orb. Moll. p. 72, 



no. 640 -Do. Cuba Moll. p. 41, no. 498. 

 ?=Lucina cselata, Hve. Conch. Ic. pi. 6, f. 27, a, b. 



A small colony was found in a burrow in Chama, old and 

 young, most tightly wedged. They apparently lived on till 

 they smothered each other, The shape of individuals varies 

 extremely. Some are nearly globular, others (smaller) much 

 flattened. Some are solid, some with a glossy shell like 

 Kellia. In some the ligament was conspicuous outside ; in 

 others entirely hidden. The outside is sometimes nearly 

 smooth ; sometimes with concentric lines of growth, here and 

 there granulose. The ligamental plate is generally more or 

 less turned in ; the teeth rather divergent and strongly bifid. 

 The muscular scars are rather long, especially 'the posterior 

 one, serrated within, and (with the pallial line) near the mar- 

 gin ; but even these characters seem to vary in every specimen. 

 The umbo in the very young shell is subcentral, with the liga- 

 ment entirely external, and the hinge teeth prominent. The 

 smallest specimen measures '04 across ; the largest, long. '41, 

 lat. '42, alt. '28. 



After a very careful comparison of the types of L. cselata, L. 

 orbella and D. semiaspera, I am unable to detect differences 

 between them which do not exist between specimens of the 



