AHT. 16. NEW MUSSELS FROM MEXICO AND URUGUAY — MARSHALL. 3 



of Colonia, Uruguay, and was collected and presented by Dr. Floren- 

 tino Felippone, of Montevideo. 



The delicate nature of the shell may be judged by considering its 

 size in connection with the combined weight of the two valves, which 

 is only 0.8346 gram, or approximately one thirty-fifth of an ounce. 

 The species does not seem to bear any close relationship to any of 

 the other species of the genus, its nearest relative being Diplodon 

 {Bulloideus) hulloides Lea, of which the type is in the collection of 

 the United States National Museum. Perfragilis is readily distin- 

 guished from huUoidens by its fragile nature, its more elongated 

 form, the absence of a markedly angular posterior ridge, and the 

 pinkish cast to the whole shell. The radiating bars on the umbones 

 are very much stronger than is usual in Diplodon and lead one's 

 thoughts to the genus Castalina. Further investigation with more 

 material available, especially the soft parts which, unfortunately, are 

 lacking in this specimen, may show that a new subgenus or section 

 is needed for it. At present the section Bulloideus affords the best 

 resting place for it. 



DIPLODON PODAGROSUS, new spe ies. 



Plate 1, figs. 1, 2, 5 ; plate 2, figs. 6, 7. 



Shell inflated, thick, subelliptic in outline, broadly rounded and 

 subtruncate posteriorly, more narrowly rounded anteriorly. Hinge 

 line lightly arched, joining the anterior margin in a curve with no 

 perceptible angle, joining the posterior margin in a rounded angle. 

 Sculpture of rather rude growth ridges with finer concentric lines 

 between them and obscure indications of crude radial sculpture. 

 Posterior ridge high, broadly rounded. Posterior dorsal area broad 

 with a shallow indistinct groove running from the beak to the middle 

 of the posterior margin. Beaks eroded, high, and full. Ventral 

 margin gently curved, appearing to be slightly cut away anteriorly 

 Periostracum thick, dull, of a nearly uniform dark chestnut brown. 

 Interior white with a pale flesh tint and with but little iridescence 

 except at the adductor and pallial scars. Of the two pseudocardinal 

 teeth of the right valve the inner one is long and high and sharply 

 serrated on its upper edge. The outer one is a low, thin, long, indis- 

 tinct plate. The single lateral tooth of this valve is long and high 

 and spatulate, its edge sharply crenulated, and its faces granulate 

 and obliquely striate. The single pseudocardinal of the left valve is 

 thin, high, serrate, and obliquely striate, its inner base standing on a 

 little shelf. Of the two lateral teeth in this valve, the outer one is 

 slightly the lower and the shorter, its edge sharply serrated. Edge 

 of inner tooth lightly serrated. The two faces of these teeth facing 

 each other are granularly, obliquely striated. 

 5596— 24— Proc.N.M.voI.6.S 28 



