MARYLAND GEOLOGICAL SURVEY 461 



This species is a coninioii form in tlie Maryland and Virginia Miocene. 

 Seo'uenza records it in tlie Miocene of Calabria and d'Orbigny described 

 it from the Vienna Basin Miocene. Tt l^ecomes more abundant in the 

 later Tertiary. 



Occurrence. — Choptaxk For^iiatiox. Jones Wharf. C^\lveht For- 

 MATiox. Chesapeake Beach. 



dollrction. — ]\raryland Genlooical Survey. 



Genus POLYSTOMELLA Lamarck. 



The shells of this 1)eautiful and delicate genus consist of regular, 

 equilateral, nautilus-sha])ed, convolute type in which but the final con- 

 volution is visible externally. The complex structure of the interior 

 of the chambers is admirably worked out and portrayed by Dr. Carpenter 

 in his Introduction to the Study of the Foraminifera. Prof. Brady in 

 the Challenger Eeport l)rietly but clearly defines the genus as follows: 

 " The test of Polystomella is, as a rule, of lenticular or discoidal form. 

 In the weaker modifications (e. g., Polystomella striato punctata) the seg- 

 ments are more or less inflated, and the external furrows by which they 

 are separated are bridged over at intervals by extensions of the inner 

 margins of the segments, leaving rows of depressions or ' fossettes ' to 

 mark the septal lines. These marginal extensions of the segments are 

 called ' retral processes ' or in connection with their external shelly 

 investment ' septal bridges ' and throughout a considerable section of the 

 genus their presence to a greater or less extent is the only advance in 

 structure upon that of the Xonionlncv:'' This author adds that in 

 more typical forms the septa are limbate externally and the retral 

 processes develoj) into a series of transverse ridges which almost or 

 completely connect the septa of contiguous chambers. It is this feature 

 which characterizes the Miocene forms of the Maryland deposits. Dr. 

 Uhlig records it as early as the Middle Jurassic but it is not well repre- 

 sented until late Tertiary time. We have but one specimen from the 

 Miocene of Maryland but in the overlying Pleistocene of Cornfield Har- 

 bor further southward it becomes the most abundant foraminifera of the 

 region. 



