390 CEETACEOUS PELECYPODA 



rocks of Kansas is another typical species of this genus. The marked inequality 

 of the valyes, the general flatness of the right one, and its narrow deep byssal sinus 

 with a very small anterior ear justify a generic separation from the last named 

 genus, but it seems to me that its characters should be a little enlarged, and that 

 species like Avicula Munsteri, Bronn, for which Meek proposes the name Oxytoma^ 

 should not generically be separated from them fvide Check-list cret. and jurass. 

 fossils. Smith. Misc. Coll., No. 177, 1864, p. 39). There is indeed a direct passage 

 from the posteriorly scarcely winged JPsetidomonotis speluncaria to Avicula echinata, 

 Mlinst., from this again to A. Munsteri, Bronn, then to Avic, costata and incequi- 

 valvis and others, the latter being called typical Oxytoma by Meek. The genus is 

 probably represented already in Devonian deposits by such species as JPterinea 

 bifida, Sandberger, and it continues up to the close of the cretaceous period. 



12. Cassianella, Beyrich, 1862, (Zeitsch. d. deutsch. Geol. Gesellschaft, 

 vol. xiv, p. 9; Laube, in Denksch. Akad.,. Wien, vol. xxv, pt. ii, p. 46, et seq. ; 

 GrypJiorhynchus, Meek, Am. Journ. of Sc. and Arts, 1864, vol. xxxvii, p. 217). 

 Very inequivalve, left valve convex, with strongly incurved beaks, with a small 

 triangular anterior and a shorter blunt or truncate posterior ear, right valve gener- 

 ally equal or slightly smaller, flattened or somewhat convex, with small indistinct 

 ears on either side; hinge line straight ; hinge in the left or inflated valve consist- 

 ing of a few small cross-teeth below the beak and a long rib on either side, parallel 

 to the hinge line ; ligament situated in a long groove running from the apex poste- 

 riorly and somewhat obliquely to the margin of the hinge line ; hinge of right 

 valve apparently similar to that of the left ; surface smooth or radiately ribbed ; 

 type, Cass. grypJiceata, Miinst., sp. (AviculaJ. The same species has been selected 

 by Meek as the type of his GryphorhyncMis, and besides that the author (loc. cit., 

 p. 218,) suggests a sub-generic name Actinophorus for the radiately ribbed species 

 like Cass, decussata, Miinst., but it is clear from Laube 's analyses and figures of 

 the hinge of this and other allied species that there is no ground for a further 

 generic or sub-generic separation, and both of Meek's names must, therefore, be 

 considered as synonyms of Cassianella, 



13. Fteroperna, Morr. and Lye, 1853, (Moll Great OoL, pt. ii, p. 16). Sub- 

 equivalve, moderately inflated, obliquely oblong ; hinge line straight, with a very 

 small anterior wing and a broad shallow insinuation below it, posterior wing long, 

 narrow; ligamental groove sub-external, shallow, long, extending from the beak 

 posteriorly; hinge with numerous small oblique teeth below the umbones and 

 one or two long ribs posteriorly, more or less parallel to the margin of the wiDg ; 

 anterior muscular scar small, close to the anterior ear, posterior elongately ovate, 

 large, excentric; type, Ft, costulata, Deslongsch. sp. All the known spegies are 

 from Jurassic rocks. 



14. Aucella, Keyserling, 1846, (Petschora-land, &c., p. 297). Obliquely 

 elongated, inequivalve, of thin structure, pearly within and with concentric sul- 

 cation externally, left valve strongly convex with incurved beaks, with a short 

 posterior and an almost obsolete anterior ear, represented by a slight internal 



