328 [December, 1843. 



Observations. 



In 328 species of organic remains already described, independent of minute 

 and uncertain forms, 43 have been found in a recent state; and when to these 

 are added numerous very small shells which are found in every careful examina- 

 tion of marl from the various localities, the percentage of recent species may be 

 pretty accurately estimated at about 11 or 12 per cent. This result I have ob- 

 tained by a careful exploration of the fossilliferous beds, and two visits to the 

 shores of Florida and Alabama to procure recent forms for comparison with 

 those of the Miocene strata. The last trip furnished three recent shells before 

 known only in the fossil state. 



Several species described in the preceding pages I obtained during a recent 

 excursion to Petersburg, Virginia, at the kind invitation of Mr. Tuomey. In 

 the course of a few hours examination of the marl in the vicinity in three or 

 four days, I collected about 100 distinct species. This locality is peculiarly 

 interesting, being the western limit of the Miocene, having a considerable eleva- 

 tion above tide, and based on granite, it is the spot in which to search for the 

 estuary and fresh water shells of the Miocene period. Mr. Tuomey has already 

 found an extinct Cyrena, and a univalve not very unlike* Paiundina sub- 

 purpurea, (Say,) whilst I discovered a species of Cerithium, identical with a 

 shell which I found last spring inhabiting oyster beds in the Manitee river, 

 near its junction with Tampa Bay. It is about the size and nearly resembles 

 C. trilineatum, figured in Kiener's work. 



The elevation of the Petersburg Miocene is considerably more than 100 feet 

 above tide, and as the rise decreases towards the sea, it is probable that the pri- 

 mary rocks continued to be uplifted even after the era of the Miocene ; indeed 

 how can we otherwise account for the elevation of fossilliferous beds, even of 

 those of the Post-Pliocene period ? 



It is an interesting fact that the Miocene estuaries were inhabited by two 

 species of bivalves, now extinct, of the same two genera which still occur in simi- 

 lar situations in Florida and Alabama, that is at the confluence of rivers and bays, 

 where the water is nearly fresh. These genera are Gnathodon and Cyrena, 

 both of the family Ctrenad.tj. The extinct Gnathodon has a considerable 

 resemblance to the recent species, but the Cyrena is widely different from the 

 living shell. These fossils are frequently water-worn, always with disunited 

 valves, and appear to have been transported. Occasionally a specimen occurs 

 not in the least abraded, a circumstance which indicates the vicinity of the 

 Petersburg deposits to the mouth of a river. The strata occur in a meadow, 

 and consist of blue marl of a sandy texture, often intermixed with small gravel, 

 and of ferruginous sand, full of shells ; there is here also a proportion of gravel, 

 of rounded quartz, occasionally of large size. Water-worn fragments of 

 bivalves are abundantly intermingled with entire shells, and many species occur 

 with connected valves. This is particularly the case with the burrowing shells, 

 as Panopoea, but also, though less frequently, with the large Venus tridacnoides, 

 Crassatella undulata, Astahte concentrica, Cytherea albaria, two species 

 of Chama, and even two species of Ostrea are not uncommon ; but there is 

 nothing like an oyster bed in these strata which might indicate shoal water. The 



P. glaber, (Terbo glaber, Lea.) 



