1880.] NATURAL SCIENCES OF PHILADELPHIA. 27 



On the other hand, the fossils of the newer deposits as exhibited 

 in the sections on the west bank of the Fatuxent show a verj' 

 decided similarit>' to those of St. Mary's, for out of tlie twenty- 

 two species of bivalves occurring there, no less than eleven ( or 

 just 50 per cent.) are also common to the last mentioned locality. 

 There can, therefore, I believe, be no reasonable doubt that the 

 deposits exposed on the Patuxent River immediately above the 

 Pernn beds constitute a direct continuation of the highly fossil i- 

 ferous strata bordering both sides of the St. Mary's River. These 

 last number among their fossil fauna also about thirty-four species 

 of Lamellibranchs, the same number as is found in what we have 

 designated as the older group, but of these thirty-four, about 

 twenty -two (or, deducting Corhula cuneata and Madra ponderosa, 

 twenty), or 65 per cent, are peculiar to the locality. Moreover, 

 of the entire number, about nine (or 27 per cent.) are still living 

 on the Atlantic coast. The dissimilarity of the two faunae cannot 

 fail to strike the least observant investigator, and Conrad has 

 dwelt at some length upon this curious manifestation (A. J. 

 Science, vol. xxviii, p. 282, and Bull. National Institution, 1841, 

 p. 176). That paleontologist singularly enough (apparently not 

 having made any exact numerical estimates either of the living- 

 forms, or of the forms found in one locality and not in the other) . 

 explains the differences as due solely to variable local conditions.' 



difference being a tendency on the pai't of the latter to lose the full solidity 

 of its ribs some distance before they reach the posterior slope. The V. 

 alveata exhibits a similar tendency, but not quite to the same extent. 



I have been unable to discover any description of the PJiolas ovalis. 

 Con., nor is there any mention made of it either in the Mioqene check 

 list prepared by Conrad in 1862, (Proc. A. N. S.), or in that of Meek, of 

 1864 (Smithsonian Miscell. Collections). I have, therefore, only doubtfully 

 referred it to P. costata. 



' Thus he states (A. J. S. loc. cit.) : "If our coast were now suddenly, 

 elevated, we should find spots where the shells would consist chiefly of an 

 immense number of Modiola demissa mixed with Littorina littoirea and 

 Melampas bidentatus ; these are found on the margm. of the lagoons at 

 high water mark, the Modiola imbedded in a tenacious soil. At a little 

 distance would be found Venus mercenaria, Mya, avenavia^ Solen ensis, 

 Solecurtus Caribeus ; among these would be Ostvea. . ViTginiana, Fusus 

 cinereus, and a few of Pecten concentricus. Such is the group existing on 

 the sandy shore of the Estuaries. Hai'd by, would be a vast deposit of 

 oyster shells with Echinus, and immense masses, of Sevpula. These live 

 on the bottom of the lagoons, which is composed of a mixture of sand and 



