306 Systematic Paleontology 



terminating with the hirgest growth in the Upper Oriskany in R. muscu- 

 losa. 



Occurrence. — Orisklvny' Fohmatiox, Eidgeli' ]\Iember. Knobly, 

 Williams Eoad, and Collier's Kun, near Cumberland, Maryland ; Pendle- 

 ton County, West Virginia. 



Collection. — Maryland Geological Survey. 



Ehipidomella musculosa var. akctisinuata n. var. 

 Plate LV, Pigs. 31, 22 



Description. — Associated with R. musculosa are a number of small 

 individuals with a narrow ventral sinus, stronger medially elevated dorsal 

 valve, and an emarginate anterior edge. In many ways these shells recall 

 R. emarginata of older formations, but are more coarsely striate, thicker 

 shelled, and more strongly biconvex. They can hardly be regarded as 

 direct descendants of R. emarginata, since the Oriskany material grades 

 into R. musculosa. However, the Lower Devonian Rhipidomellas are 

 closely related forms, and it may be that R. musculosa var. arctisinuata\ 

 is reversional or atavistic in its expression. 



A very large specimen apparently of this species in Mr. Boeder's col- 

 lection measures 35 mm. in length and 40 mm. in width. The average 

 specimen is 15 mm. by 18 mm. 



Occurrence. — Oriskany Formation, Eidgely Member. Cumberland. 



Collection. — U. S. National Museum. 



Ehipidomella marylandica n. sp. 

 Plate LV, Fig. 23 



Description. — This species has the external expression of R. assimilis or 

 half-grown individuals of R. musculosa, with which it has been con- 

 founded. It differs from all Lower Devonian Ehipidomellas in the narrow 

 ventral muscle area and the nearly complete absence of the raised lateral 

 and anterior margins usually seen in this area in other species. The 

 ventral valve is depressed convex as in R. musculosa, but has not the broad 

 shallow sinus of this species. Dorsal valve unknown. 



Length 2.5 cm. ; width 2.5 cm. 



