Paleontology Zorritos Formation 19 



sectional characteristic of Ractina. They are smaller in size 

 than the usual Miocene Raetas. 



The Corbulas of the section Aloidis present transitions 

 between other similar forms from Antillean horizons, feat- 

 ures which are brought out in the systematic discussion of 

 the group. This genus, with six species, in two sections, 

 stands next to the Areas in point of specific diversity. 

 Corbida (Cnncocorbula) acutirostra n. sp. is a sharply 

 defined species with an unusually marked development of 

 the posterior keel, and is most closely related to two species 

 from the Oligocene of Germany. 



A rapid survev of the faunal list confirms the natural 

 expectancy of finding a tropical facies, and it is hardly worth 

 while to use space for detailed discussion of that point. 

 Almost all of the genera thus far described from the region 

 are characteristic of the tropical seas, and none is extrane- 

 ous to such a habitat. It is true that some of the types have 

 been found in cooler-water faunas, but such have a wider 

 range, and their presence in this fauna introduces no dis- 

 cordant element. 



The preponderance of ecologic evidence ofifered by the 

 fauna, and supported by the type of sediments in which they 

 were entombed, indicates a shallow water habitat as the 

 rule. All of the species are of genera now living in the 

 littoral zone, most of them present in the recent fauna of 

 that zone on the Peruvian coast, an excellent list of which 

 has been published by Dall.^* Some of the matrix verges on 

 the conglomeratic in character, containing pebbles of the 

 sort one would expect to find in somewhat agitated waters, 

 and hardly in the quiet of the usual ofT-shore bottom. Most 

 of the species, however, occur in sediments distinctlv of the 

 sandy littoral type, and it is fairly certain that the fauna 

 flourished, for the most part, in that zone. 



" Proc. U. S. Nat. M115.. vol. 37. Xo. 1704. pp. 147-294. pis. 20-28, 

 1909. 



