28 PROCEEDINGS OP THE ACADEMY OF 



It would hardl}' have been tliouglit a Corbula by those who insist 

 on the group being referred to that genus, if no other species of 

 Pachyodon liad been found. 



P. erectus, Conrad. 



I have not the specimen originally figured, of this species, to 

 refer to, and cannot say positively whether P. alius, Conrad, is 

 identical with it. There is no form in the collection which would 

 represent P. erectus. 



P. altus, Conrad, PI. I., figs. 4, 18. 



These represent the right or small valve of P. altus in its 

 3'oung stage of grovvth. There are many specimens of this species 

 in the collection, but not one form which I could refer to the pre- 

 ceding species. 



P. cuneata, Conrad, PI. I., fig. 3. 



This figure represents an unusual form of this shell, which 

 species is rare in the collection in comparison with P. tenuis, oh- 

 liqua, or erectus, about 10 specimens in all. 



Subgenus Anisorhyncus, Conrad. 



One valve of this subgenus occurs in the collection, the principal 

 distinctive character of which is the involution of the right valve. 

 What gives peculiar interest to this shell is that it is closely re- 

 lated to A. py}'iformis. Meek, which lived among Unionidas in the 

 eocene of Utah, and the subgenus unknown in later tertiary de- 

 posits. Tins is another instance of the belated character of the 

 South American faunas, and favors the belief that the Pebas 

 group is not of late tertiar^^ origin. 



p. (Anisorhyncus) cuneiformis, Conrad, PI. I., fig. 19. 



Pyriform, very inequilateral, ventricose anteriorl}', with the sub- 

 margin slightl}'^ convex, and the area before it depressed ; posterior 

 side elongated, somewhat sinuous or contracted, end acutely 

 rounded ; umbonal slope undefined. 



This description applies to only one right valve, which is all in 

 the collection. It is a moderatel}- thick shell, and differs from 

 most species of Platyodon in wanting a prominent beak. 



