680 GEOLOGY OF OHIO. 



thus giving them the appearance of being turned backwards, The 

 ' subreniform shape is peculiar, as is also the unusual fullness of the pos-. 

 terior half. The anterior muscular scar is well marked but the posterior 

 one is quite indistinct in the specimen. 



Formation and Locality : Middle or Modiolodon beds of the Trenton, 

 formation, near Burgin, Kentucky. 



Ctenodonla cingulata, Ulrich. 



Plate .48. Figs. 10-12. 



TelHnomya cingulata, Ulrich, 1879, Jour. Cin. Soc. Nat. Hist., vpl. ii, p. 23. 



The types of this species do not preserve the hinge teeth clearly so 

 that they were misrepresented in the original description and figures, 

 Better examples have since been obtained and the two now illustrated 

 on plate 48 give a reliable idea of both the external and internal char- 

 acters of the shell. The hinge teeth are very slender and crowded in 

 the central part of the hinge. Their great length and the unusual width 

 of the hinge plate are the principal peculiarities of the species. 



Formation and Locality: Upper beds of the Cincinnati group, 

 Boyle and Oldham counties in Kentucky, and Dayton, Ohio. 



Ctenodonla perminuta, n. sp. 



Plate 46. Figs. 11-14. 



Shell very small, commonly about 1.5 mm. in length and 1.15 mm. 

 in height, widest anteriorly; anterior outline rounded below, generally 

 nearly vertical in the upper half, and subangular in the antero-cardinal 

 region; basal margin broadly convex, posterior end obliquely truncate, 

 strongly rounded in the lower half. The species is known from casts of 

 the interior only. In these the beaks are prominent, moderately 

 incurved, and situated between one-fourth and one-third of the entire 

 length behind the anterior extremity. The muscular impressions are 

 unusually distinct for so small a shell, and the structure of the hinge 

 seems to have been very much as in C. levata, Hall, sp., and other forms 

 of that section of the genus. 



The extreme minuteness of the shell and the comparative strength 

 of the muscular attachments, will distinguish this form at. once from all 

 other species of the genus having a similar shape. The C. obliqua Hall, 

 sp., another minute species of the Cincinnati rocks, and of which the 

 recently proposed Palceoconcha faberi (S. A. Miller, 1892, N. A. Geol. 

 and Pal., p. 498), seems t6 me to be a synonym, is a higher shell and 

 doubtless belongs to quite a different Section of the genus, being closely 

 related to such forms as C. compressaTSlrich. and C. astartceformis Salter. 



Formation and Locality : Lower and middle beds of the Cincinnati 

 group, Cincinnati, Ohio. 



