Maryland Geological Survey 343 



Pentamerus galeatus Hall, 1859, Nat. Hist. N. Y., Pal., vol. ili, p. 257, pi. xlvi, 



fig. 1; pi. xlvll, fig. 1, 1861. 

 Sieberella galeatus Hall and Clarke, 1893, ibidetn, vol. viii, pt. ii, p. 24G, fig. 



175, pi. Ixxii, figs. 7-13. 

 Gypidula galeata Schuchert, 1897, Bull. U. S. Geol. Surv., No. 87, p. 226. 

 Gypidula galeata Weller, 1903, Geol. Surv. N. J., Pal., vol. ill, p. 279. 



Description. — " Sliell varying from ovoid to subglobose and transversely 

 elliptical. Ventral valve gibbous, becoming in old shells very ventricose 

 in the umbonal region ; a strong mesial fold along the center of the lower 

 half of the valve : beak ventricose, arched, and strongly incurved over that 

 of the opposite valve. Dorsal valve often nearly circular or transversely 

 elliptical, gibbous above; area with or without a defined mesial sinus 

 towards the lower margin : beak incurved, and filling the triangular fora- 

 men beneath the beak of the ventral valve. 



" Surface, in extremely young shells, smooth, or marked only by con- 

 centric lines of growth; in older foniis, having longitudinal plications 

 more or less developed, or rarely with none. Old shells variously plicated ; 

 the plications simple or bifurcating, and crossed by fine concentric stria3 

 of growth, which sometimes become stronger imbricating laminte towards 

 the margin of the shell. 



" Internally the dental lamellae are developed into a long spoon-shaped 

 cavity or chamber, which forms a continuation from the triangular fora- 

 men or pit beneath the beak of the ventral valve. A central septum ex- 

 tends from the beak of the ventral valve, half the length of the shell, and, 

 in its upper part, is united to the conjoined dental lamellae or V-shaped 

 chamber of this valve. In the dorsal valve the two septa extending from 

 either side of the beak are attached to the shell for about half the distance 

 to the base, below which point they become free." Hall, 1859. 



Length about 3 cm. ; width about 2.6 cm. 



These shells have been invariably identified as Dalman's Atrypa 

 galeatus, but as the American specimens differ in many minor characters 

 from the Silurian specimens of Gotland, it seems advisable to recognize 

 this difference by a distinct name. The adult specimens from near 

 the top of the Coeymans formation attain a far larger size and are wider 

 anterior to the mid-length than the full-groAvn individuals from Gotland. 

 In the latter specimens the dorsal valve is devoid of plications in the 



