32 Bulletin 39 304 



The Dosinia acetabulum- Q.on.r2A. is one of the most abundant 

 and characteristic species of the Chesapeake Miocene and in its 

 course through the Chesapeake formations, several varieties are 

 developed, of great use in correlation. The typical acetabulum 

 is however confined to the Upper Chesapeake formations and it 

 was figured b}' Conrad in his Medial Tertiary, from a fine large 

 example from Smithfield, Virginia. It is characterized b y its 

 large size, broad outlines and even, concentric banding. This 

 Upper Chesapeake or typical acetabulutn corresponds very close- 

 ly in size, form and sculpture to the Costa Rican examples. It 

 is very common at certain localities in Costa Rica, especially in 

 the lower part of the Gatun formation. Bose records it from 

 Mexico. The Petaloconchus sculpturatus is a widely distributed 

 species in the West Indies and Caribbean Miocene, from which 

 it is more generally recorded as P. do^ninigensis, a name given to 

 the Santo Domingian shell by Sowerb5^ Close comparative 

 study of a large collection of typical sculpturatus from Virginia 

 and North Carolina, failed to show any real difference, either in 

 details or sculpture or in nature of coiling. The Petaloconchus 

 sculpturatus occurs in the Bowden beds of Jamaica; the Gurabo 

 formation of Santo Domingo; the Quebradillas limestone of 

 Forto Rico; the Springvale beds of Trinidad; and the Gatun of 

 Panama and Costa Rica. It is also known from Cuniana, Vene- 

 zuela: Cartagena, Colombia; and Santa Rosa, Mexico. In the 

 Chesapeake Miocene, it is restricted to the Murfreesboro, York- 

 tov\^n and Duplin formations, that is, to the Upper Chesapeake. 

 It does not occur in the overlying Pliocene so far as is known. 



5. Possibly more significant because of their greater num- 

 ber are the following species, which appear for the first time in 

 the Atlantic coast Miocene in the Chesapeake formations. They 

 also continue into the Pliocene and generally into the recent 

 fauna. 



