246 Systematic Paleontology — Middle Devonian 



anterior to the beak longer, larger and higher than the posterior part; 

 hinge and ventral margins of anterior part subparallel and anterior end 

 conspicuously rounded ; posterior part of shell contracts rapidly from the 

 beak toward that extremity which is obliquely truncate; well defined, 

 angular ridge extends from the beak to the post-basal extremity and the 

 shell slopes rather rapidly from it to the hinge-line; valve moderately 

 convex which is most marked in middle part. Surface marked by rather 

 coarse, clearly defined concentric striae which after crossing the umbonal 

 ridge continue to the hinge-line parallel to the posterior margin. 



Only a single external impression of a right valve is contained in the 

 Maryland Collection, but this is quite well preserved. It is somewhat 

 similar to Ctenodonta (Tancrediopsis) subcontracta Beushausen,* from 

 the Lower and Middle Devonian of the Ehine. The similarity at least is 

 so marked that is very evident it may be referred to this subgenus of 

 Beushausen. In Beushausen's work Ctenodonta is a genus compris- 

 ing a large number of species under which Palaeoneilo Hall emend., Tan- 

 crediopsis and others appear as subgenera. In the writer's opinion 

 Palaeoneilo is clearly entitled to generic rank and he would raise Tanr- 

 crediopsis to the same position. In regard to the orienting of the shell, 

 as to which end shall be called anterior and which posterior, Beushausen's 

 description of C. subcontracta and Hall's of a similar shell Tellinomya = 

 Ctenodonta nasuta ^ supplemented by Salter's later account ^ have been 

 followed. 



The specific name is given in honor of Dr. J. M. Clarke. 



Length, 28 mm.; height, 11^ mm. 



Occurrence. — Eomney Formation, Hamilton Member. Western 

 Maryland. 



Collection. — Maryland Geological Survey. 



^Abhand. d. Kon. Preuss. geol. Landesanstalt, N. F., Heft 17, 1895, p. 94; 

 Atlas pi. viii, figs. 14-16. 



- Pal. N. Y., vol. i, 1847, p. 152. 



' Geol. Surv. Canada, Figs, and desc. Canadian Org. remains, Dec. 1. 1859, 

 p. 36, pi. 8, figs. 1, 2. 



