382 -ZV^EJF TBIASSIC UNIOS FROM TEXAS— SIMPSON. 



Triassic. Some of these fragments were described by Meek as Unios/ 

 but they were iu such bad condition that even a generic determination 

 coukl hardly be considered certain. As the shells on which this paper 

 is based are, I believe, undoubted Uuios,and as it seems to be pretty well 

 established that the strata in which they were found are Triassic, I 

 think I need have little hesitation iu saying that these are the earliest 

 authentic specimens known of this common and widely distributed 

 genus. I may further add that in the opinion of Dr. Charles A. White ^ 

 it is quite probable that the Gallinas Creek fossils belong to the 

 Jurassic. 



Taken as a whole, these Unios closely resemble in form and are appar- 

 ently nearly related to those of the Jurassic beds of Korth America, and 

 to certain species of our Cretaceous and Tertiary formations. Thej^ can 

 hardly be said to be very near relatives of any species at present living 

 in the New World, though JJnio anodontoides and one or two other allied 

 species from the Mississippi basin have characters in common with 

 some of them. In Europe, however, the well-known Unto jjictorum and 

 other somewhat similar species, as well as most of the forms found in 

 Asia Minor, show a considerable resemblance to some of these species. 



It is remarkable that there has been so little change in the species 

 of this genus from the time when they lived in this great Triassic lake 

 to the present day. In some cases specific descriptions of these fossils, 

 wbose age probably dates back well toward the beginning of the Meso- 

 zoic, so far as all the characters which remain are concerned, would 

 apply almost without change to species living in the Euro-Asiatic region 

 to-day. And Dr. White has shown that the same persistency of char- 

 acters is true of a number of the forms of the Laramie group of the 

 Cretaceous, which iu all probability are the ancestors of some of our 

 characteristic recent Mississippi Valley species, and which can hardly 

 be separated from them.^ 



As he has pertinently remarked, these earliest types of Unios have 

 continued almost unchanged until the present, while to-day there is 

 not a single family of vertebrates in existence that lived in Triassic 

 times. This wonderful persistence of JJnio forms, and the variety of 

 cliaracters displayed in the species herein described, go to show that 

 the genus must have been well established at the time the Dockum 

 beds were deposited, and that it undoubtedly had its origin at a much 

 earlier period, thus tending to overthrow the theory of Neumayr,^ that 

 the Unionidie were derived from the genus Trigonia, which probably 

 does not date back to a period earlier than that of the shells under 

 consideration. 



' Unto cristonensis, Meek, Auu. Kept. Expl. and Siirv. West of One Hundredth Merid- 

 ian, 1875, p. 83. 



"A Review of the Non-marine Fossil Mollusca of North America, p, 425, 1883. 



^A Review of the Non-marine Fossil Mollusca of North America, p. 428, 1883. 



^Sitzungsher. d. Ic. Akad. Wiss. Wien, Math.-uaturwiss. CI., XCVIII, 1889, Heft 

 1-3, 1. Abth., p. 5. 



