28 PROCEEDINGS OP THE ACADEMY OF [1880. 



This interpretation might very satisfactorily account for the phe- 

 nomenon as far as generic distribution alone is concerned, and, 

 indeed, it would even hold good in its bearings on a limited number 

 of species, but it would hardly apply to a case such as the present 

 one, where the specific dissimilarity is so vast in such a compara- 

 tively very limited geographical extent. 



Now, if the supposition that the deposits in question were 

 deposited at two different periods be a correct one, and paleon- 

 tological evidence goes far to prove that they were, we should 

 naturally expect to find also some direct stratigraphical evidence 

 afforded by the superposition of the strata themselves.^ The 

 following section was obtained by Conrad at a point on the Chesa- 



mud. Then would be found another group of shells which live only in 

 deep water, the Astarte lunulata, Nucula limatula, N. proxima, Cardita 

 borealis, Pholas costata, in company with great numbers of Mytili. This 

 deposit we should recognize as having been formed in harbors, like those 

 of Newport and Charleston. ..." 



It will be observed, that in the above conception Conrad has confined 

 himself entirely to generic and not specific disti'ibution. 



' It may as well be remarked, that, although in the foregoing examina- 

 tion of the molluscous fauna I have dwelt exclusively upon the Lamelli- 

 hranchiata, the Gasteropoda offer equal, if not greater support to the 

 general conclusion arrived at. On comparing the lists of geographical 

 distribution given by Conrad in the Bulletin of the National Institution 

 (pp. 181-7), it will be seen, that not a single recent form occurs among 

 the eighteen enumerated from the Calvert cliffs at "Hance's;" and 

 further, that only two species, Valuta mutabiUs and V. solitaria, are 

 common to the forty-two found at St. Mary's. Of these last eight (or 19 per 

 cent.) were considered by Conrad to be recent forms : 



Buccinum trivittatum, Natica duplicata, 



" lunatum, Dentalium dentalis, 



" quadratum, Fusus cinereus, 



Natica heros, Scalaria clathrus. 



Nearly all the species found on the west bank of the Patuxent also 

 occur at St. Mary's, and the same can be said of those collected in Calvert 

 county near Cove point the southern extremity. Singularly enough, that 

 although three species of Turritella — T. indenta, T. exaltata, and T. 

 perlaqueata—yf ere cdWected from Calvert cliffs in the upper portion of the 

 county, none of them appear to have been found near Cove Point, where 

 " vast quantities " of a new species, T. plebeia, "the common species of 

 St. Mary's River" .(loc. cit. p. 182), appear suddenly to make their 

 appearance. 



