Paleontology Zorkitos Formation 23 



tween the faunules of the Upper and Lower Zorritos forma- 

 tions which may well prove constant in face of further 

 collecting, and may thus serv-e as a valuable guide to the 

 recognition of the units where fossils are found and there is 

 doubt, owing to structural complexities, as to the identity of 

 the beds. This difference appears most clearly expressed in 

 the Areas and the Turritellas, because of the abundance of 

 those forms and their marked difference in facies. The 

 Areas of the Upper Zorritos formation, as has been noted, 

 are larger, heavier shells than those of the Lower Zorritos, 

 and a similar difference occurs in the Turritellas. Separate 

 species of other single genera do not occur in both forma- 

 tions, as far as the present collection shows. 



The difference in the Areas may be an index to a greater 

 time dift'erence between the L'pper and Lower Zorritos forma- 

 tions than the general affinities now apparent seem to demand. 

 The Areas of the L'pper Zorritos appear to be of a later type, 

 some of them closely related to A. idonea Conrad, a species 

 which is most abundant in the Chesapeake Miocene, in beds 

 of Tortonien age. Others are close to A. grandis Broderip 

 and Sowerby, and A. tuberculosa Sowerby, both more 

 modern types. Cerithimn Grillannm n. sp. is very close to 

 C. nisselli Maury, from the Helvetien of Santo Domingo, 

 and two of the Turritellas, T. nelsoni and its variety rotiiii- 

 data are close to species from the Helvetien of Europe. 



These later affinities, despite the fact that they are over- 

 shadowed in present number by the strictly Burdigalien 

 affinities, promise a greater possibility of finding more evi- 

 dence of their kind than there is for the Lower Zorritos 

 faunule, which in its larger number contains a proportionally 

 much smaller Helvetien. and Tortonien eleinent. Thus, 

 although it is not possible to say that the L'pper Zorritos is 

 very much younger than the Lower Zorritos, it is advisable 

 to indicate the definite possibility that future work may 

 reveal a Helvetien element sufficiently strong to necessitate 

 considering the fauna of that age. In the present state of 



