LOWER SILURIAN LAMELXIBRANCHS. 687 



he says "there are no traces of an external ligament to be seen." My 

 specimens show clearly a narrow dorsal groove between the edges of the 

 valves which I believe to have lodged a ligament. 



As to the use of McCoy's genus Anodontopsis for this group of shells, 

 I would say only this, that before such a cause is justifiable it must be 

 shown that the type of his genus really has the same kind of hinge and 

 muscular impressions as is above ascribed to Cycloconcha. Meek's name 

 Orthodontiscus, which he suggested might be used for the species milleri 

 should it prove to be generically distinct from Anodontopsis, has no claim 

 to recognition at the expense of Miller's name, since it was never de- 

 fined nor used as a genus by anyone. 



It is very difficult to determine the family relations of this genus. 

 The hinge is quite different from all paleozoic shells, but agrees rather 

 well with those of the recent genera Cyrena and Crassatella. At present 

 however, we cannot say that this resemblance indicates natural relation- 

 ship, since Cycloconcha is, so far as known, restricted to the Cincinnati 

 group, and not a single shell is known from rocks succeeding that forma- 

 tion as high up as the Lias, that might be regarded as connecting the 

 Lower Silurian genus with those recent genera. It may be that such con- 

 necting links may yet be discovered, if indeed the}' have not already been 

 lound and are misunderstood, but until we do know something better, I 

 propose to classify Cycloconcha as a peculiar family by itself. 



Cycloconcha ovata, n. sp. 



Plate 48. Figs. 13-15. 



Shell about 17 mm, long, 12.3 mm. high, and 7.2 mm. thick; outline) 

 excepting the slightly prominent beaks, regularly oval; occasionally the 

 posterior end seems to have been a little higher than the anterior. Sur- 

 face covered with such fine concentric lines that unless viewed through a 

 strong lens, it appears perfectly smooth. The only specimen on which 

 these surface markings can be detected also shows in the middle of 

 the posterior cardinal slope six or seven obscure lines radiating from 

 the beak. Hinge strong, with the cardinal tooth of the right valve 

 triangular, only a little oblique and distinctly triplicate. Posterior lateral 

 teeth of same valve three in number, the central one the strongest. 

 Anterior lateral teeth of right valve three, the middle one the most prom- 

 inent, the upper one formed by the margin of the valve, the lower one 

 curved, shorter, and not as prominent as the central one. In the left 

 valve the cardinal tooth is about half the size of the cardinal tooth of the 

 right valve and situated in front of the pit into which that tooth entered. 

 Of lateral teeth there are four, two anterior and two posterior; both pairs 

 strong and subequal. Muscular scars not satisfactorily observed. As 

 exhibited in a cast of the interior they appear to be less distinctly im- 

 pressed than in C milleri Meek, sp. 



