504 Systematic Paleontology 



Amadropsis compacta n. sp. 

 Plate XIII, Figs. 3, 4 



Description. — Shell small, squat, relatively heavy; whorls live in num- 

 ber, increasing quite rapidly in size, narrowly tabulated, those of the spire 

 subtrapezoidal in outline, the body evenly and quite strongly inflated; 

 external surface smooth excepting for oblique incrementals ; sutures deeply 

 impressed; aperture holostomous, a little more than half the altitude of 

 the entire shell, obliquely elliptical in outline ; the outer lip a little more 

 strongly arcuate than the inner and the anterior extremity a little more 

 broadly rounded than the posterior; peristome continuous, the outer lip 

 probably thickened, the inner lip reflected over the body wall and almost, 

 but apparently not quite, closing the umbilicus ; umbilical chink probably 

 narrow but deep ; columella reinforced in front of the umbilicus. 



Dimensions. — Altitude 7.5 mm., maximum diameter 5.9 mm. 



Type Locality. — McNeys Corners, Prince George's County. 



The type, the only specimen in which the shell is preserved, is imperfect 

 at the umbilicus so that it is impossible to tell some of the critical char- 

 acters with assurance. The shell is rather heavy for Amauropsis, but in 

 the totality of characters it seems to be closer to that genus than to 

 Eusprira. The form differs from Amauropsis meekana Whitfield in the 

 squat outline and much more inflated body whorl. 



Occurrence. — Monmouth Formation. ? Brightseat, Friendly, ? 2 

 miles southwest of Oxon Hill, Prince George's County. 



Collection. — Maryland Geological Survey. 



Family TROCHEIDAE 



Genus MARGAR1TES Gray 

 [Ann. Mag. Nat. Hist., vol. xx, 1847, p. 271] 



Shell thin, nacreous, trochiform or turbinate, whorls inflated, well 

 rounded, as a rule, and usually few in number; external sculpture de- 

 veloped, dominantly spiral; suture lines impressed or channelled; aper- 

 ture subcircular or obliquely produced ; peristome not continuous ; outer 



Etymology: iiapyaplr-qs, a pearl. 



