i8 Studies in Geology, No, 3 



uncertainly located Nelson collection, being in very good 

 condition, having served best for morphologic study ; the 

 poorer material gives hints of some variations both in size 

 and form which > may deserve systematic recognition, but 

 not enough can be substantiated to make distinctions. The 

 Dosiuiiuac of the collection are further characterized by the 

 occurrence of a large, heavy form, Dosiiiia grandis Nelson, 

 very much like D. orbicularis Agassiz, of the European 

 Miocene. 



Perhaps the most interesting single occurrence in the 

 pelecypod fauna is that of Aniiaiitis iiicrassota var ovoidalis 

 Sacco, a variety common in the Tongrien of Europe, and of 

 a genus very rare in known American faunas. It is hardly 

 possible, in view of the several variations of A. incrassata 

 in Europe, that this particular variety reached Peru as a 

 direct migrant from the central European seas, and the ap- 

 parent absence of the species in well known faunas occurring 

 along i>ossible lines of migration and at the same general 

 time horizon lends strength to that assumption. It seems 

 far more likely that parallel development may account for 

 the evolution of characters in the Peruvian form exactly 

 similar to these of the European incrassata. 



The Raetas are always an interesting element, and the 

 Zorritos fauna, with three well-defined yet closely related 

 species, presents some unusual developments in that group. 

 Labiosa (Ra'cfa) gardnerac occurs in great profusion in the 

 Upper Zorritos sandstone at Ouebrada de las Alturas, and 

 one specimen has been found in the Lower Zorritos. L. ven- 

 tricosa is a short, inflated shell unlike any known fossil 

 Ra'cta, and of very interesting aspect. Some of these small 

 fossil Raetas show affinity to the section Ractina Dall,'^ of 

 recent Indo-Pacific distribution, but they all show evidence 

 of the posterior lateral tooth, the absence of which is a 



"Trar.s., Wag. Inst., vol. 3, pt. 5, p. 



