1921] NATURAL SCIENCES OF PHILADELPHIA, 31 



of Texas by C. T. Simpson,^ and from the Connecticut Valley by 

 Emerson and Troxell.'s All of these authors referred their species 

 to the northern genus Unio; but Simpson subsequently^" decided 

 that at least part of the Texan species were related to South Amer- 

 ican genera, though he did not expressly alter the generic references. 

 A list of American Triassic species follows: 



Species from the Dockum beds, Staked Plains of Texas: Unio 

 subplnnatus, Unio dumblei, Unio graciliratus, Unio dockumensis, all 

 of Simpson. 



Species from Trias of the Connecticut Valley: Unio emersoni 

 Troxell, Unio wilhrahamensis Emerson. 



The third and fourth species, at least, might better be referred 

 to the genus Diplodon, having radial beak sculpture. Probably 

 the others will also prove to have similar sculpture when material 

 sufficiently well preserved comes to hand. 



In three of the York species the radial sculpture of the beaks 

 shows relationship with South American genera, being like that of 

 Diplodon and Hyria, and totally unlike that of Unio and allied 

 genera of the Northern Hemisphere. These species are herein re- 

 ferred to the genus Diplodon. In two other York species the beaks 

 themselves are not preserved well enough to make out their sculp- 

 ture; but as there is corrugation of the posterior slope, it is likely 

 that the beaks are radially folded and that these also are referable 

 to Diplodon. 



The genus Mycetopoda, here recognized as a Mesozoic fossil for 

 the first time, is a South American genus belonging to the Mutehdae, 

 a family of the southern continents. 



The records for Triassic Unionidae are as yet few; but the wide 

 separation of the localities, the presence of several species and 

 their considerable diversity in shape and sculpture in each area, 

 may permit the inference that Triassic North America possessed 

 a large and varied Naiad fauna of South American type, Hyriinae 

 and Mutelidae. The next fauna of these mussels of which we have 

 any definite knowledge is that of the Jurassic in Colorado and Wyo- 

 ming. Here the South American types have entirely disappeared, 

 and in their place are distinctive^ Holarctic Unioninse, in which 



^* Description of four new Triassic Unios from the Staked Plains of Texas. 

 Proc. U. S. Nat. Mus., 1895, pp. 381-385. 



" American Journal of Science (4), vol. 38, 1914, p. 460. 



2" Synopsis of the Naiades or Fresh-water Pearly Mussels, 1900. 



