58 PALAEONTOLOGY OP THE UPPER MISSOURI. 



exceedingly interesting to see how gradually they shade off through the smooth, 

 subplicate, and plicate Myophorias of the Trias, with their more developed hinge, 

 into the highly ornate Jurassic, Cretaceous, and living Trigonias, with their still 

 more complex dental system. 



Genus SCHIZODUS, KING. 



Synon.Telllnites (sp.), SCHLOT. Akad. Munch. VI, 1816, 31. 



Axinus (obscurus), SOWEEBT, Min. Con. IV, 1821, 12. 



Isocardia (axiniformis), PHILLIPS, Geol. York. 1836, 209. 



Cucullsea (Schlotheimi), GEINITZ, Neues Jahrb. 1841, 638. 



Donaxf (sulcatus), J. DB C. SOWEEBT, Geol. Tr. V, 1840, 491. 



Sedgwickia (gigantea), McCov? Garb. Foss. Ireland, 1844, 62. 



Dolabra' (part), McCoy, Carb. Foss. Ireland, 1844, 64. 



Myophoria, McCoy, 1855, Palaeozoic Foss. Great Brit. 1855, 494 (not BRONS, 1855). 



Schizodus, KINO, Ann. Mag. Nat. Hist. XIV, 1844, 313. 

 Etym. fx}&i I split ; oJouc, a tooth. 

 Type. Schizodus truncatus, KIHG. 



Shell more or less oval or subtrigonal ; anterior side rounded and shorter than 

 the other; posterior side tapering, more or less truncate at the extremity, and 

 usually having an umbonal ridge extending from the beaks to the postero-basal 

 margin. Beaks generally prominent. Surface smooth, or ornamented with con- 

 centric striae. Hinge with two smooth cardinal teeth in the right valve, and 

 three in the left ; the middle tooth of the left valve being more or less bifid, and 

 fitting between the two of the opposite valve. Free margins smooth. 



Animal unknown. 



As pointed out by Prof. King, this genus is closely related to Myophoria, of 

 Bronn. Prof. McCoy has also subsequently expressed the opinion that the slight 

 differences observable between the teeth of the hinge in these two groups are not 

 of generic value, and ranges the species under Bronn's name Myophoria. 2 As we 

 have had no opportunity to examine specimens showing the hinge, we merely retain 

 provisionally, the name proposed by Prof. King. We would remark, however, that 

 so far as we can judge from figures alone, we are inclined to think that the non- 

 plicated Carboniferous and Permian species, with possibly a few of the similar 

 forms from the Triassic rocks, should be retained under Prof. King's name, either 

 as a distinct genus, or as a subgcnus under Myophoria. 



Prof. King has also called attention to the close relations between tnese two 

 groups and the genus Trigonia ; the principal difference between them, so far as 

 the hinge is concerned, being the greater development, and sulcated character of 



1 As proposed by McCoy, the genus Dolabra was made to include species belonging to two distinct 

 families. The first or typical species, Cucullsea angusta, Sw., and three others included by McCoy, 

 constitute the genus Dolabra as properly restricted, by the separation of the species upon which Prof. 

 King founded the genus Schizodus. As thus restricted, the genus Dolabra belongs to the Arcidx, 

 and seems to be closely allied to Cucullsea, Lamk. Dolabra? alpina, Hall, Iowa Report, I, part 2, 

 p. 716, pi. xxix, fig. 2, is a true Schizodus, and its name should be Schizodus alpinus. 



3 British Pal. Foss. p. 494. 



