'174 SYSTEMATIC PALEONTOLOGY 



and siibtriangular outline of its later chambers. In the variety " alata " 

 there is a well defined wing or keel around the periphery and the test is 

 rather more flattened than in the BoUvina beyrichii types. 



The prolongation of the aperture, together with its marginal keel, 

 furnishes a sure key to the identification of the species. It is found in 

 existing seas at depths from 50 to 800 fathoms. 



Occurrence. — Calvert Formation. Chesapeake Beach. 



Collection. — Maryland Geological Survey. 



Family LAGENID/E. 

 Subfamily NODOSARIN/E. 



Genus CRISTELLARIA Lamarck. 

 The genus Cristellaria is represented in its typical form by a plano- 

 spiral lenticular shell with the aperture always at the outer margin 

 of the periphery. Sometimes, however, the later chambers of the shell 

 become enlarged and drawn out so that the shell becomes very oblong 

 and when the primary chambers are very small and the later ones ex- 

 tremely developed it resembles the jSTodosarian type. The genus makes 

 its first appearance in the Triassic. It is very widespread at the present 

 time and occurs at all depths but is most common at depths of less than 

 300 fathoms. 



Cristellaria cultrata (Montfort). 

 Plate CXXXII, Fig. 15. 



Rohulux cnltratuK Montfort, 1808, Conch. Syst., voL i, p. 314, 54:tb g-eure. 

 CrMellaria cultrata Bagg, 1898, BulL Amer. Pal., No. 10, p. 26. 



Description. — Test circular, biconvex, smooth and glistening; margin 

 sharp and broadly keeled; chambers seven to eleven in the last volution, 

 somewhat convex, either smooth or costate ; aperture radiate. The width 

 of the marginal keel is very variable, though always more or less devel- 

 oped, and this constitutes the essential feature of the species. 



Occurrence. — St. Mary's (?) Formation. Crisfield Well (7T6 feet). 



Collection. — Maryland Geological Survey. 



