54 CRETACEOUS PELECYPODA 



distinctive character by which they could he generically separated. 11. and A. Adams 

 quote Bla'mvilUa, Hup 6, 1854, (non idem E,ohijieau-Desvoidy, 1830,) as identical 

 Avith Mactr'mula. Chenu, however, considers Blainvillkc as a distinct genus, wliich 

 externally appeals to me to he very similar to dementia, particularly looking at 

 the species which he figures as Bl. vUrea, Hup 6. In any case if that shell really 

 belongs to the mactrin^, and is sufficiently different from Mactrinula and Ilarvellu, 

 it must receive a new name. 



18. Cymhophora, Gabb, 18G9, (Pal. Calif., vol. II, p. 180), proposed for Mactra 

 Ashhurnerii a cretaceous species. Form typical as in Mactra, but " the hinge is 

 composed of a rather heavy hinge-plate, bearing a cartilage-pit, not sunk into its 

 substance, as. in other genera of the Mactridm, but, as it were, built upon its surface ; 

 a small delicate spoon-shaped process laid obliquely under the beaks, its base being 

 on, or slightly above, the level of the hinge-plate ; in the right valve the cardinal 

 tooth is single, very delicate, and nearly at a right angle with the anterior wall of 

 the cartilage-pit ; in the left valve the tooth is A -shaped, entirely separated from the 

 pit, very slender, and articulates between the tooth and the pit of the opposite side ; 

 the lateral teeth are large and comparatively very robust." The cartilage-pit is 

 certainly peculiar (? natural), but whether this is sufficient reason for a generic 

 separation seems to me doubtful. 



19. Sinsiila, Gvdij, 1837, f^emmwc^ra. Swains., 1840). Shell svib-trigonal ; 

 cardinal teeth short, strongly elevated and emai'ginated in the left valve, small hi 

 the right one ; lateral teeth diverging, long, cross-ribbed ; single in the left, double 

 in the right valve ; ligament marginal, not sex)arated from it by a shelly ridge ; 

 pallial sinus moderately deep and horizontal. 



19a. The more elongated triangular forms were called by Morch Oxyperas, 

 like Sp. fOxyp.J tnangidaris, Lam. 



20. Mactra, Linn., 1767 fTrigonella a^ud H. and A. Adams). Shell trigonal, 

 anterior cardinal tooth in the left valve strong, in the right smaller, lateral teeth 

 long, considerably raised in the middle ; ligament separated from the cartilage-pit 

 by a shelly ridge ; pallial sinus moderate, roundish or sub-angular. The two follow- 

 ing divisions may be considered as sub-genera of Mactra. 



20a. Schizodesma, Gray, 1837, (Trigonella, daCosta, auctorum,) has been 

 proposed for two species, M. Spenffleri and nitida, which only differ from typical 

 MactrcB by not having the ligament distinctly separated from the cartilage, but 

 this character does not appear very constant. 



21. Mulinia, Gray, 1836. The cardinal teeth are in both valves strong, and 

 the ligament is almost quite internal, attached to the inner edge of the margin, 

 just above the cartilage, from which it is not separated by a shelly plate. 



22. Pseudocardiiim, Gabb, 1866, (Palseont. of Calif., vol. ii, Sect, i, pt. i, p. 20). 

 " Shell thick, heavy, equivalve, resembling Lcevicardium externally, ligament inter- 

 nal, lunule cordate, deeply impressed. Hinge composed of a large cartilage-pit, 

 and in the left valve of a A -shaped tooth articulating in a corresjwnding depres- 

 sion on the opposite valve ; two lateral teeth in each valve very large and promi- 



