TRANSACTIONS OF WAGNER 

 778 



TERTIARY FAUNA OF FLORIDA 



Genus PLACUNANOMIA Broderip. 



Plamnanomia Urod., P. Z. S., 1832, p. 28. Type P. Ciiinin^ii Brod. 

 Placitiwiniii Swainson, Malac., p. 390, 1840; Gray., P. Z. S., 1849, p. 120. 



Placunanomia plicata Tuomey and Holmes. 

 P. plicata T. and H., Pleioc. Fos. S. Car., p. 19, pi. 6, figs. 4-6, 1855. 



Newer Miocene of Duplin County, North Carolina, at the Natural Well, 

 Burns ; and at Smith's on Goose Creek, South Carolina, Tuomey and Holmes ; 

 living in Charleston Harbor, South Carolina ? Ravencl. 



Tuomey and Holmes state that Dr. Ravenel had in his collection a 

 recent specimen of this species obtained from Charleston Harbor, but the 

 absence of any confirmatory evidence for more than forty years leaves the 

 accuracy of this determination in doubt. The fossil much resembles P. 

 Cumingii, but is less deeply plicated, more delicate, with the rugosities of the 

 cardinal border more feeble, and the byssal scar nearly equal in size to that 

 of the adductor, in the right valve, while in P. Cumingii the adductor scar is 

 conspicuously the larger of the two. If the present species be extinct, as 

 seems likely, it is one of several instances where peculiar forms which were 

 common to both coasts of America before the Pliocene survive the separation 

 of the two oceans only on the Pacific side, a result which I believe to be due 

 to the much steeper slope of the Pacific shores, which enabled many species 

 of mollusks or their embryos to migrate seaward as the land rose and thus 

 survive the change, while the more level margin of the Atlantic resulted in 

 the total desiccation of a wide strip of sea-bottom in a relatively short space 

 of time, thus exterminating a large proportion of the less active littoral fauna 

 simultaneously over the whole of the elevated border of the coast. 



Conrad has briefly described (Kerr, Geol. Rep. N. Car., App., p. 19, 1875) 

 an unfigured Miocene species from North Carolina under the name of P. 

 fragosa. The type is lost and the generic place of the species is doubtful. 



Placunanomia lithobleta n. s. 



Rare in the Oligocene of Bowden, Jamaica; Henderson and Simpson. 



Shell resembling P. plicata, but flatter; not plicate, but gently waved 

 distally ; surface radially sculptured with minute, almost microscopic, threads, 

 which are frequently interrupted, when the termination of the proximal part 

 of the thread is swollen, resembling a minute head or pustule; interior re- 

 sembling P. plicata, but the hinge weaker, the amorphous irregularities con- 



