52 CHETACEOUS PELECYPODA 



8. Lahiosa, Schmidt, 1832. Shell thin, similar to the last, with a sharp rib 

 running from the beaks to the infero-postcrior margin ; anterior lateral tooth less 

 oblique than in the last genus, in other respects the hinge is very similar in both ; 

 and it would probably be better to consider this only as a sub-genus of the 

 former. 



9. Vanganella, Gray, 1819. Shell oblong, rather compressed, beaks sub-cen- 

 tral, with two ribs diverging from them internally ; hinge with the cartilage-pit large, 

 produced obliquely towards the anterior rib, the posterior cardinal tooth of tlie left 

 valve folded together, of the right valve small and separate ; lateral teeth small, 

 close to the cartilage process ; ligament marginal, not perfectly separated from the 

 cartilage. 



This genus is based upon a remarkable recent species, V. Tmjlorii, Gray. The 

 anterior internal rib is quite similar to that of the fossil Machontya (vide postea), 

 the hinge of which is, however, not known. Agassiz figured in his monograph of 

 the fossil M>/(B several species which may turn out to be Vanganellce. I allude 

 specially to such forms as Mactromya mactroides or II. Gouloni, 



10. Zenatia, Gray, 1819. Shell oblong, compressed, upper and lower margins 

 subparallel, umbones anterior; posterior gape wide; cartilage-pit large; cardinal 

 teeth prominent, anterior lateral tooth in the left valve rather distinct, posterior 

 laterals quite close and almost confluent with the upper edge of the shell ; ligament 

 sub-marginal, not separated from the cartilage. 



11. Fteropsis, Conrad. I am only acquainted with this generic form from the 

 check -list of inv. foss. of N. America, (eocene and oligoeene), Smithson. Misc. Coll., 

 No. 200, 186G, p. 8. Two species. Ft. pcqit/ria and lapidosa, are described from 

 eocene beds. 



12. Tresus, Gray, 1849. Shell oval, solid, externally perfectly similar to that 

 of Mya, widely gaping posteriorly ; hinge with a wide cartilage process, very short 

 cardinal teeth, and the lateral teeth small and situated near the cartilage-pit ; 

 ligament external, marginal ; pallial sinus very deep. The only species T. maximus 

 is frona California. 



13. Schizothcerus, Conrad, 1852 (Proc. Acad. N. S. Phil., p. 199). Is very 

 closely allied to Tresus with a deep channel on either side of the cardinal teeth. 

 The figure of the species, Sck. ( Cryptodoti) NuttalUl, given on pi. 18 of vol. vii., 

 Journ. Acad. Nat. Sci. Phil., 1837, indicates the presence of very thin anterior lateral 

 teeth. The terminations of the siphons of Schizothcerus are protected by two soUd 

 valves, which I am not aware of having been noticed in Tresus. In every other 

 respect both genera could be very easily identified. The fossil species, Schizothoirus 

 Nuialli* Conrad, described in vol. V of Reports of expl. and surveys to ascertain 

 the ... route for a railroad, &c., p. 321, pi. iv, fig. 33, is considered by Gabb to be 

 the same species (Pal. Calif., vol. II, p. 91), but it shows no trace of lateral teeth. 

 Unless this is explained by a change of age, in which case the latter would be a full 

 grown form of the former one, I cannot see the reason for the identification. 



* I do not know exactly which tlie proper spelling of the two words is ; they are almost in every instance, 

 referred to, writt«n in a different way hy American authors themselves. 



