ART. 1 NEW LAND AND FEESH-WATER MOLLUSKS MARSHALL 5 



It came from "Five miles northeast of Dockum, head of Duck 

 Creek, Dickens County, Tex." Unio gradliraius Simpson,^ p. 384, 

 text fig. 4, and Unio dockumensis Simpson,^ p. 385, text fig. 5, belong 

 in this new genus. The fourth species described by Simpson, Unio 

 subplanatus Simpson,^ p. 383, text figs. Nos. 1 and 2, is a large fresh- 

 water mussel but does not fall into Antediplodon and can hardly be 

 an Elliptio ( Unio) . 



At the time Simpson's paper was published the old classification of 

 the pearly fresh-water mussels was still in use, and the importance 

 of beak characters had not yet been fully recognized, hence his use 

 of the generic name Unio for all of them. He points out the resem- 

 blance of the beak sculpture to that of some of the South American 

 naiades. Had the new classification been in use it is probable that 

 Simpson would have done as is being done in this paper; that is, he 

 would have formed a new genus for the three species mentioned 

 above as belonging to Antediplodon. This new genus became neces- 

 sary to receive the species described below. 



ANTEDIPLODON DEWITTENSIS, new species 



Plate 1, Figure 7 



Shell wedge-shaped, having its greatest diameter near the anterior 

 end; tapering somewhat and having its least diameter at the pos- 

 terior extremity. Beaks set far forward, about 7 mm. behind the 

 extreme front end and about 40 mm. in front of the extreme rear 

 end. Posterior dorsal margin nearly straight; ventral margin slightly 

 curved, apparently joining the posterior margin in a blunt point and 

 sharply rounding into the anterior margin. Posterior ridge lacking; 

 anterior ridge very high, rounded. Anterior area nearly truncate, a 

 rather large lunule near the beaks. 



The type (Cat. No. 371003, U.S.N.M.) measures: Length, about 

 47 mm. (estimated, because a portion of the rear end is lacking); 

 height, about 24 mm.; greatest diameter, about 24 mm. 



This species is well characterized by its donaciform shape. The 

 common east American marine shell Donax fossor Say if greatly en- 

 larged would have about the same form. The species is not closely 

 related to any other known species, recent or fossil. Its nearest rel- 

 ative is Antediplodon dumhlei Simpson {Unio dumhlei). 



POLYGYRA MYERSI, new species 



Plate 1, Figures 5, 10 



Shell with spire depressed-conic; base full and rounded; whorls 

 six, flattened, narrow except the last which is moderately broad. 

 Sutures well marked; periphery somewhat angulate; sculpture of 



' Simpson, Description of four new Triassic Unios from the Staked Plains of Texas. Proc. U. S. Nat. 

 Mus., vol. 18, no. 1072, 1896, p. 383, text fig. 3. 



