Paleontology Zorritos Formation 47 



uniformly three subsidiary threads, of which the middle 

 one sometimes approaches the strength of a major thread. 

 Aperture oval; details not known. Height, 26; diameter, 

 14.5 mm. 



The closest relative of this species is P. polygomis var. 

 percostata Sacco,*" from the Lower Miocene of Europe. It 

 is not tA.'pical of the common Photinae, the pronounced keel 

 giving the whorls a squarish aspect rather unusual to the 

 group, and in the absence of detailed knowledge of the 

 aperture it is not completely certain that it belongs in the 

 genus ; however, there is small doubt, since all visible char- 

 acters agree very well wath those of P. polygomis, which is 

 one of Cossman's plesio-types of the genus Phos s.s.*^ It is 

 a well-characterized species, and appears to have no close 

 relatives in the American Tertiar\'. It differs from P. 

 polygomis var. percostata Sacco in having broader longi- 

 tudinal folds and a sharper keel which is more nearly level 

 with the suture than in percostata. The sculpture of latiru- 

 gatus is more diverse. In general shape the two are close. 



The spiral sculpture commonest to P. latirugatus, that 

 mentioned in the description, is not similar to that of 

 polygomis, but some mutants of the Peruvian species show 

 a coarser sculpture which does agree fairly well with that 

 of the variety percostata figured by Sacco (loc. cit.). These 

 mutants may prove to deserve recognition as separate vari- 

 eties of latirugatus, but as far as the material now available 

 shows they are sufficiently close to the type to make differ- 

 entiation inadvisable. 



Lower Zorritos. Quebrada Zapotal. 



" Piemonte, pte. 30, p. 58, pi. 14, figs. 60, 61, 1904. 

 *^Essais de Pal. Comp., vol. 4, p. 158, 1901. 



