MARYLAND GEOLOGICAL SURVEY cxlix 



We may, in considering the fauna of the Maryland Chesapeake, find 

 the analogy of its location under present conditions in either of two ways. 

 We may consider the present geographical distribution of the genera 

 represented in it, or we may take the surviving species and consider their 

 present distribution on our coasts. The latter is the most definite 

 method leaving less to the judgment of the statistician. From twenty 

 to twenty-five species survive from each of the Maryland horizons. Of 

 these eleven at present extend from the existing boreal fauna to the 

 subtropical waters of Florida, and therefore afford no more precise 

 indication. Of the remainder seventy per cent now live in the fauna 

 existing from Hatteras southward, while only thirty per cent are con- 

 fined to the region from Hatteras northward. We may therefore con- 

 clude, (1) that the temperature conditions governing the fauna of the 

 Maryland Chesapeake were those of the temperate rather than the boreal 

 or subtropical faunas of the present coast; and, (2) that the temperature 

 of the Chesapeake embayment was on the whole somewhat warmer than 

 at present. This is what the genera represented also indicate. Between 

 the several horizons of the Maryland Chesapeake there is but very slight 

 indication of any temperature difference; so far as there is an}^, it points 

 toward a progressive but slight cooling of the water from the Calvert 

 to the St. Mary's; while the subsequent Pliocene was doubtless accom- 

 panied by a change in the opposite direction, a rise of temperature being 

 indicated by the changes in the fauna. 



It has been shown ' that the shell-bearing mollusk fauna of the cool- 

 temperate zone comprises normally about 400 species in any reasonably 

 diversified area. When thoroughly done, collecting from marl beds will 

 give much better results as regards completeness than can be had from 

 any dredging in the actual sea, because the marl is so much more acces- 

 sible than the seabottom. In the Maryland Miocene, omitting varieties, 

 about three hundred and sixty-four species are recorded. It may be 

 supposed that about forty species remain to be discovered in the Mary- 

 land beds. 



Of the species known about ten per cent are supposed to survive. 

 This small number is partly the result of the rather restricted limits 



3U. S. Geological Survey, Bulletin 84, pp. 35-31, 1892. 



