159 MURFREESBORO STAGE — OlvSSON 



shell, variability of form and obsolescence of its sculpture. 



The best and most widely known of our Miocene fossils is 

 the peculiar and interesting gasteropod Ecphora quadricostata. 

 This species commences first in the Choptank, where it is repre- 

 sented by a widely umbilicated form, generally separated as the 

 variety umbilicata of Wagner. In the St. Mary's beds the spe- 

 cies becomes typical, continues up into the Murfreesboro and 

 Yorktown stages, where it is large and often abundant. It is 

 pre-eminently a cold water or Chesapeake species, and during 

 Murfreesboro time, as will subsequently be shown, followed the 

 Chesapeake fauna south into South Carolina and into Florida. 

 During the milder portion of the Upper Miocene, the species 

 persisted in the cooler portions of its range, that is, in the 

 Yorktown basin north of the then already defined Hatteras axis. 

 It is lacking from the more southern, warmer, but synchronous 

 Duplin beds. 



To the Murfreesboro stage belongs the distinction of having 

 more species of Pectens than any other of our Miocene stages. 

 Of the commoner Maryland species, all are represented except 

 the Lower Miocene Pecten humphreysii and marylandicus. Pecten 

 jeffersonius and madisonius have continued up from the Dower 

 and Middle Miocene. Several species are peculiar. The Upper 

 Miocene Pecten eboreus, makes its first appearance, becomes com- 

 mon and the dominant form in the Yorktown and the Duplin 

 stages. Nearly confined as a fossil to the Murfreesboro and char- 

 acteristic of it is Pecten clintonius, which in some localities is ex- 

 tremely abundant. The species is very rare in younger beds, 

 but evidently persisted to the present day, the recent P. magel- 

 lanicus , by many being considered identical. The species which 

 are grouped around P. virginianus and decemnarius are nearly 

 peculiar to the stage and are good index fossils. 



Another group of the monomyarian pelcypods, abundant in 

 the Murfreesboro stage, are the oysters, represented by about 

 two species, namely O. disparilis and sculpturata. These two 

 species are not known in Maryland, but are common and wide- 



