213 Systematic Paleoxtology — Middle Devonian 



incurved and covering that of the opposite valve; apex perforate; the 

 interior shows two strong teeth; the muscular impressions begin at the 

 base of the rostral cavity, and continue to a little below the middle of 

 the length of the valve; outside the muscular impression the surface is 

 often variously marked by vascular impressions. Dorsal valve gibbous, 

 mucli more convex than the opposite valve; umbo prominent; outline 

 regularly convex above the middle and curving abruptly to the sides; the 

 usually undefined mesial fold becomes visible below the middle of the 

 valve, and usually very conspicuous toward the front, which is abruptly 

 elevated. Surface marked by concentric lines of growth, the lamellae 

 often closely imbricated and sometimes nearly wanting near the umbo 

 but crowded toward the front of the shell; well preserved specimens occa- 

 sionally marked by fine interrupted and scarcely distinct radiating striae. 



The majority of the Maryland specimens are internal impressions ; but 

 there are a few in which the shell is more or less perfectly preserved. 

 A ventral valve of the latter closely agrees with the more oval form of this 

 species from the Hamilton formation of iSTew York and there are internal 

 impressions very similar to the ventral one figured by Hall from Hardy 

 County, Va.* The specimens are not so gibbous as those from the cal- 

 careous shales of western jSTew York, and perhaps this difference is partly 

 due to crushing in connection with the folding of the Maryland rocks. 

 The species is distinguished by the size, shape, greater convexity of dorsal 

 valve, broad and shallow sinus on anterior part of ventral valve which is 

 often deep near the margin forming a nasute front; conspicuous surface 

 lamellae; and the outline of the muscular impressions on the interior 

 of the valves or internal impressions. 



Length, 32 mm. ; width, 33 mm. 



Occurrence. — Romney' Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; B. & 0. E. R. cut at 21st Bridge; west 

 of iron bridge over Town Creek nortlieast of Oldtown. 



Collections. — Maryland Geological Survey; New York State Museum; 

 American Museum of Natural History. 



» Pal. N. Y., vol. iv, pi. xlvi, fig. 29. 



