4 PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



Psoronaias Crosse and Fischer, of which a couple of species are found 

 living in northeastern Mexico, namely, Psoronaias herrerae Marshall 

 and Psoronaias semigranosus von dem Busch, and several other species 

 in Guatemala. In Psoronaias the beak ornamentation is indistinctly 

 of the nested V pattern and the undulations are broken into numerous 

 granules. Both genera have riblets on the posterior area running 

 from the ridge to the margin. 



The beak ornamentation is related to that of some of the Naiades 

 of the upper Cretaceous formation of Wyoming and Utah. 



EONAIAS REYNOSENICA. new species 



Plate 1, Figures 3, 4, 6 



Shell nearly quadrate, inflated, somewhat oblique. Dorsal margin 

 arcuate, making nearly a right angle with the posterior margin. Ven- 

 tral margin well rounded, curving abruptly, almost angularly, into the 

 posterior margin, and slopingiy rounding into the anterior margin, 

 which is regularly curved and offsets at its upper part to allow for a 

 large lunule under the beak. Beak set well forward, about 10 mm. 

 back of the extreme front end and 32 mm. in front of the extreme rear 

 end. Posterior ridge high, roundly angular. Anterior ridge not so 

 well marked, but the anterior area making a rapid descent from the 

 disk of the shell to the margin. Beak with V-shape undulations, 

 " nesting" one within another, those distant from the beak tending to 

 break into granules, especially on the anterior area. Posterior area 

 with a number of riblets running from the posterior ridge to the 

 margin. Concentric sculpture strong. Cavity of the shell deep, the 

 anterior adductor scar deeply punched. Pseudocardinal teeth mas- 

 sive, laterals thick and short. 



The type (Cat. No. 371001, U.S.N.M.) measures: Length, 42 mm.; 

 height, 33 mm. ; diameter, if both valves were present, would be about 

 24 mm. Other specimens (Cat. No. 371002, U.S.N.M.) have about 

 the same proportions as the type. The largest measures: Length, 

 48 mm.; height, 38 mm.; diameter, if both valves were present, would 

 be about 34 mm. 



In the type some of the beak characters are almost worn away 

 and indistinct. In some of the paratypes they are very well preserved^ 



ANTEDIPLODON, new genus 



Characterized by elongate form, abrupt anterior end, and especially 

 by the sculpture of the beak, which consists of several fine, clear-cut, 

 direct, radiating riblets. 



The type is TJnio dumhlei Simpson.^ (PL l,figs. 2,8.) 



'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. 



