THE LOWER SILURIAN ROCKS. 643 



the Cincinnati group and contains O. casei, O. fissicosta, Meek, sp., and 

 probably O. alternata, all three having small and bifurcated costse, and 

 the greatest height of the shell in the anterior half. The other group, 

 comprising O. notabilis, O. laticostata, and O. illinoisensis {Ambonychia 

 illinoisensis, Worthen), occurs in the middle beds of the group and possi- 

 bly in the upper as well. It is distinguished from the casei section by 

 the coarsely plicated surface and by the more or less decided posterior 

 position of the line of greatest height. Yet it seems to me a case rather 

 of undiscovered connecting links than one of distinct affinities. In evi- 

 dence of this view I would cite the finely striated but posteriorly high O. 

 obliqua, which links closely enough to O. alternata, and the coarsely pli- 

 cated and centrally highest O. ampla. There is, I grant, much room for 

 intermediate forms, but I expect confidently that diligent search will 

 sooner or later close the gaps. 



The great length of the hinge line is the character now chiefly relied 

 upon in distinguishing the genus from Byssonychia, Anomalodonta and 

 Eridonyckia. 



Opisthoptera casei, Meek and Worthen. 



Plate 49, Figs. 1-5. 



Ambonychia (Megaptera) casei, Meek and Worthen, 1866, Proc. Chicago Acad. Sci.' 



Vol. I. p. 23; also 1868, Geol. Sur. 111., Vol. Ill, p. 337. 

 Not Ambonychia (Megaptera) casei, M. and W. ?, 1873, Ohio Pal., Vol. 1, p. 133 (O. 



fissiscosta, Meek sp.) 



The figures of this species on plate 49 will give, it is believed, a 

 better idea of both its internal and external character than has been pre- 

 sented heretofore. They are also clear enough to render a detailed de- 

 scription unnecessary. 



Formation and localities : Good specimens are always rare, and so 

 far as known the species is restricted to the upper fifty feet of the Cin- 

 cinnati group. More or less imperfect casts of the interior are not un- 

 common at Richmond, Indiana, and are occasionally met with at 

 Waynesville and Clarksville, Ohio. A good mold of the exterior (see 

 plate 49, fig. 1) was collected near Lebanon, Marion county, Kentucky. 



Opisthoptera fissicosta, Meek. 



Plate 49, Fig. 15. 



Ambonychia (Megaptera) casei? Meek, 1873, Ohio Pal., Vol. I, p. 133 (The name 

 fissicosta is suggested as appropriate should the peculiarities of the species 

 described prove constant).* 



* It is unfortunate that Meek so often shirked the responsibility of the erection 

 of a species by merely suggesting a name that may be used should the characters 

 observed by him prove to be constant or of sufficient importance to merit recogni- 

 tion. An injustice was thereby perpetrated upon those who followed him, and who 

 accepted the responsibility that he would not, for they, though not really obliged 

 to adopt his name are still in a measure bound to do so. Such a proceeding may 

 show a cautious mind, but it is a kind of caution that is scarcely to be recommended, 

 since it secures credit that is not deserved. 



