LOWER SILURIAN LAMELLIBRANCHS. 679 



trie lines between them. Escutcheon rather narrow but long and deep. 



This shell though larger, is exceedingly like the geologically earlier 

 W. compressa Ulrich, from the lower Trenton of Minnesota, and might 

 well be considered, if not a reapparition, a variety of that species. Still, 

 as the Ohio shell is somewhat narrower across the posterior half and rep- 

 resented by casts of the exterior, while the Minnesota form is known 

 only from casts of the interior, it is not unlikely that other differences 

 may be shown when we can compare better and equal material of the 

 two. Differences in their hinges and muscular impressions are to be 

 looked for since there is some evidence to show that these parts were 

 stronger in W. compressa than in W. Uhiocnsis. 



Compared with associated species, W. obliquala Ulrich is much more 

 convex and has stronger umbonal ridges; W. umbonata Ulrich is much 

 fuller in the umbones and has a different outline, W. subovatu Ulrich is 

 ovate rather than rhomboidal in outline, and W. quadrangularis Whit- 

 field, sp., is shorter and much more ventricose. 



Formalionand Locality : Upper beds of the Cincinnati group, Waynes- 

 ville and Clarksville, Ohio. 



Family 'NUCULIDyE, Gray. 



Genus CTENODONTA, Salter. 



i 

 Te/Hnomya, Hall, 1847, Pal. N. Y., vol. i, p. 151; 1857, Tenth Ann. Rep. Reg. Univ. 

 N. Y., p. 181. Not Tellinomyj., the correct form of Tellimya, Brown, 1827, as 

 given by Agassiz in his "Nomenclator Z'oologicus" in 1846. 

 CtenodonU, Salter, 1851, Rep. Brit. Asso. p. 63; 1859, Can. Org. Remains, Decade 1, 

 p. 34; Ulrich, Final Rep. Geol. Sur. Minn., vol. iii, p. 578 (in press). 

 I had prepared an entire plate of species of this genus for this paper, 

 but the lack of time required for the final study which it is my habit to 

 give to all species described by me immediately preceding the trans- 

 mission of the manuscript to the printer, has induced me to delay their 

 publication to some other opportunity^ The illustrations of the three 

 species following happened to be placed on plates devoted chiefly to other 

 shells here described, so that I could not very well postpone their con- 

 sideration. 



A full description of the genus and of numerous species will be 

 found in the Minnesota work above cited. 



Ctenodonta retrorsa, n. sp. 



Plate 50. Figs.H and 15. 



This species is founded upon the cast of the interior of a single 

 valve, probably the right. If this view is correct the beaks are situated 

 a short distance behind the center and curved toward the longer end, 



