1921] NATURAL SCIENCES OF PHILADELPHIA. 35 



of growth, and on the posterior ridge, radiating, fine folds; dorsad 

 of the ridge, on the posterior slope, the folds form a dorsally curved 

 corrugation. The ligament is strong and prominent. 



Length 34 mm.; alt. 22 mm.; diam. 19 mm. 



Another specimen (Plate II, fig. 6) from the same locahty, but 

 from a different bed, is in the conglomerate of a deep brownish 

 drab color. As usual in this rock, the surface is not well preserved, 

 but the outlines are perfect. The corrugation of the posterior slope, 

 distinct in the type, is not legible. It measures: 



Length 45.5 mm.; alt. 25 mm.; semi-diam. 9.5 mm. 



In outline, this species resembles Unio graciliratus Simpson of the 

 Dockum beds, Dickens County, Texas; but in that shell the lira- 

 tions of the posterior slope appear to run parallel to the posterior 

 ridge, not diverging upwards towards the ligament, as in the Penn- 

 sylvanian species; also they continue to the posterior end, while in 

 our species the corrugated area is much less extended. 



It is named in honor of Mr. Charles T. Simpson, whose work 

 initiated the modern classification of Unionidae. 



Locality: Shale, Little Conewago Creek (loc. 5). 



Diplodon yorkensis n. sp. Plate III, flgs. 2, 2a. 



The tj'pe is a nearly perfect specimen showing nothing suggesting 

 distortion of the original form. 



The shell is long, narrow and plump, the diameter and altitude 

 about equal. The moderately full beaks are about 28 per cent, of 

 the length from the anterior end, which is rounded. The posterior 

 end slopes obliquely, the posterior-basal angle being produced. 

 The posterior ridge is prominent, rounded, the basal margin straight, 

 the middle of the valves flat. The surface shows rather strong, 

 unequal wrinkles of growth where best preserved. On the dorsal 

 slope there appears to be some trace of fine, curved, radial corru- 

 gation posterior of the beaks. 



Length 32 mm.; alt. 11.5 mm.; diam. 11.7 mm. 



This species is quite distinct by its long, plump shape. Since 

 the above account was written a second specimen has been found. 

 In this the valves are spread partly open. Each shows a depression 

 running from the beaks obliquely downward and backward to the 

 basal margin. There are radial riblets on the beaks, though mostly 

 obliterated, and a fine corrugation on the dorsal slope behind the 

 beaks, the corrugations radiating from the posterior ridge and curv- 

 ing towards the dorsal margin, thus confii-ming the indistinct traces 

 of such sculpture shown by the type specimen. This shell is larger 

 than the type, 40 mm. long, and similar in other proportions. 



Locahty: Shale, Little Conewago Creek (loc. 5). 



