Z PROCEEDINGS OF THE NATIONAL MUSEUM vol.74 



the genus Hyria^ and consisting of several V-shaped undulations, 

 " nesting " one within the other, and other undulations on the ante- 

 rior and posterior umbonal areas, each pair of which if continued 

 .would form another V in the series. 



Type of the genus,— Prodiplodon singeioaldi, new si^ecies, de- 

 scribed in this paper. 



This genus is distinctly related to Diplodon and to Hyr'ia and 

 seems to stand about midway between them. In Diplodon the undu- 

 lations are more nearly direct radial than V-shaped. In Hyna they 

 are composed of " nested " V's but in that genus the shell is alate 

 anteriorly and posteriorly. The fossils at hand give no indication of 

 having been alate posteriorly, but in the type the anterior end is 

 minutely winged. It is possible that this genus was the forerunner 

 of both Hyna and Diplodon. Prodiplodon may have one living 

 representatiA'e in " Castalia " pazi Hidalgo,^ the type locality of 

 which is Imbabura, Ecuador. Simpson ^ places it in the genus 

 Diplodon. From the description and figure it appears to be a 

 Prodiplodon and a direct descendent of the new fossil species Prodi- 

 plodon singewaldi. It appears to be the only Diplodon-like species 

 living in the region near the localities in which the fossils Avere found. 

 True Diplodon is most plentiful in the La Plata system and in south- 

 eastern Brazil. It occurs also in Chile and eastern Brazil, a couple 

 of species in Guiana, and a couple in Patagonia. Th« abundance of 

 Diplodon^ evidently a descendent of Prodiplodon^ in the La Plata 

 system is interesting when considered in connection with Erodona 

 (apparently descended from Anisothyris, the most plentiful fossil 

 in the deposits under consideration) which is found in the brackish 

 waters of the La Plata estuary. 



PRODIPLODON SINGEWALDI, new species 



Plate 1, figs. 3, 6 



Shell rather compressed, somewhat quadrate in form, rounded in 

 front, slopingly truncated at the rear. Ventral margin a little curved 

 in its median portion, sweepingly rounding into the anterior margin 

 and making a broad angle with the posterior margin. Dorsal margin 

 arched, making a very broad, scarcely noticeable, angle with the 

 posterior margin. Beaks well forward, about 19 mm, behind the 

 extreme anterior end and 51 mm. in front of the extreme posterior 

 end. Posterior ridge low, slightly angulated at upper part of shell, 

 anterior ridge almost entirely lacking. Sculpture of very prominent 

 concentric lines marking rest periods, with minor concentric striae be- 

 tween them. Umbonal area sharply and closely sculptured with ra- 



" .Tourn. de Conch., vol. 8, p. 353, pi. 13, flg. 6, 1868. 

 •Descriptive Cat. Naiades, p. 1273, 1914. 



