390 CRETACEOUS PELECYPODA 



rocks of Kansas is anotlier typical species of this genus. Tlie marked inequality 

 of the valves, the general flatness of the right one, and its narrow deep hyssal sinus 

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

 "■euLis, hut it seems to me that its characters should he a little enlarged, and that 

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

 should not generically he separated from them fcide 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 Fseudomonotls speliuicaria to Acictda echhiata, 

 Miinst., from this again to A. Milnsterl, Bronn, then to Acic. costala and iuccqiii- 

 vcdcis and others, the latter heiug called typical Oxijtoma by Meek. Tlie genus is 

 prohahly represented already in Devonian deposits by such species as Fterinea 

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



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

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

 Gn/pI/orlii/iicJ/us, Meek, Am. Journ. of Sc. and Arts, 1864, vol. xxxvii, p. 217). 

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

 triangular anterior and a shorter blvmt 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- 

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

 to the hinse 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, f/ri/phceata, Miinst., sp. (AvicnlaJ. The same species has been selected 

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

 p. 218,) suggests a sul)-geueric name Acti)wpliorus for the radiately ribbed species 

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

 the hino-e 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; ligamcntal groove sub-external, shallow, long, extending from the beak 

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

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

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

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

 from Jurassic rocks. 



14. Aiicella, 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 



