.Maryland Geological Survey 505 



lip thin, sharp and strongly arcuate body wall heavily glazed; columella 

 non-plicate ; umbilicus profound, rarely closed by a reflected layer of 

 callus; not margined by a crenulated umbilical keel. 



Margarites is initiated apparently in the Upper Cretaceous and still 

 maintains its minor position in the molluscan faunas. The genus is best 

 represented to-day on the west coast of North America, where it has been 

 subdivided by Ball into four sections, one of which is characteristically 

 boreal, another temperate, the third warm-temperate and tropical, and the 

 fourth abyssal in distribution. 



A. Whorls three in number, spire relatively low Margarites depressa 



B. Whorls four in number, spire of moderate altitude. . . .Margarites abyssina 



C. Whorls five in number, spire relatively high, altitude and maximum 



diameter approximately equal Margarites elevata 



Margarites depressa n. sp. 

 Plate XIII, Fig. 6 



Description. — Shell small, thin, nacreous, depressed, a little more than 

 thrice coiled ; whorls increasing regularly and rapidly in size, somewhat 

 flattened behind, strongly rounded in front ; external surface lineated with 

 exceedingly tine, moniliform spirals restricted to a very thin external layer 

 which is for the most part decorticated ; aperture holostomous, circular 

 in outline; body whorl rounding smoothly into the umbilical area; umbili- 

 cal pit rather small but profound. 



Dimensions. — Altitude 1.75 mm., maximum diameter 2.5 mm. 



This species is conspicuous among the Maryland Upper Cretaceous rep- 

 resentatives of the genus for its small size and depressed spire. The type 

 is unique. 



Occurrence. — Monmouth Formation. — Brightseat, Prince George's 

 County. 



Collection. — Maryland Geological Survey. 



Margarites abyssina (Gabb) Meek 



Solarium abyssinus Gabb, 1861, Proc. Acad. Nat. Sci., Phila., for 1860, p. 94, 



pi. ii, fig. 9. 

 Margarita abyssinus Meek, 1864, Check List Inv. Fossils, N. A., Cret. and 



Jur., p. 18. 

 Margarita abyssina Conrad, 1868, Cook's Geol. of New Jersey, p. 728. 



