Maryland Geological Survey 385 



towards front. Dorsal valve convex, flattened in middle. Surface orna- 

 mented by numerous rounded striae of which about six occupy 5 mm. 

 Striae obsolescent towards umbo. Shell substance punctate. 



Length and width of large shells 4 cm. 



The generic relationships of this species are not fully detennined. It 

 is probably a Eensselaeria. 



Occurrence. — Heldeeberg Formation, Keyser Member. Cut of 

 Baltimore and Oliio Eailroad f mile southwest of Rawlings, abundant. 



Collection. — Maryland Geological Survey. 



[Swartz.] 



Subgenus BEACHIA Hall and Clarke' 

 KENSSELvIiRIA (BeACHIA) PROAVITA n. Sp. 



Plate LXVII, Figs. 1-3 



Description. — This very early Beachia is distinguished from R. cum- 

 herlandicB, to which it is closely related, in being from one-half to two- 

 thirds smaller, more square shouldered and less elongate, and the dorsal 

 valve is flatter. 



At first it was mistaken for E. mutabilis, but since it has the nearly 

 obsolete radial striations restricted to the anterior margins, a flatter dorsal 

 valve, and is more square shouldered or transverse, it could not be left 

 with this species. The great similarity of R. proavita to R. cumberlandice 

 and R. mutabilis indicates that these species are closely related and that 

 the latter is the stem species of Eensselasria. The writer has specimens of 

 R. mutabilis from the Keyser of Bloomfield, Pennsylvania, ranging from 

 mature examples 10 mm. long to young less than 5 mm. Among these are 

 three examples of R. proavita ranging in length from 5 to 9 mm. They 

 have no radial striae nor marginal crenations and in this also agree with 

 the earlier growth stages of typical R. mutabilis. These facts lead to the 

 inference that R. mutabilis develops from a smooth-shelled terebratuloid, 

 introduces marginal striae, and then variation tends in two main lines. 

 The stem branch — Eensselceria — continues the increase in number of 



^Beachia Hall and Clarke, 1S93, Nat. Hist. N. Y., Pal., vol. viii, pt. ii, p. 260. 

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