PALEONTOLOGY ZORRITOS FORMATION 89 



than in P. subangulata, and there is no shoulder. A heavy, 

 bulbous callus surrounds the lower columella, merging into 

 the peristome at its upper extremity, and terminating below 

 in a rounded, bulb-like mass. There is no umbilical chink. 

 Peristome, inner aperture, and base of columella broken away 

 in all specimens available, but the aperture is undoubtedly 

 somewhat narrow and oblique. Base rounded. Height, 34 ; 

 maximum diameter (estimated) 35-37 mm. 



This species differs from P. subangulata Nelson in being 

 lower and in the callus, which is more bulbous, completely 

 covering the umbilical region. The general conformation of 

 the upper whorls is practically the same for the two species. 

 In general shape, and in type of callus, P. porcana is similar 

 to Natica josephinia Risso, from the Miocene and later of 

 Europe. 76 In shape it is much like N. josephinia s.s., and 

 the callus is similar to that of the variety antiqua Sacco. 77 



Lower Zorritos. Between Quebradas Heath and Charafi. 



Genus SINUM Bolten 



Sinum carolanum n. sp. 



PLATE IV Fig. 10. 



Shell depressed, with a smooth nucleus of 3^ whorls, in 

 all with five or six whorls. The smooth nucleus slopes 

 abruptly, being separated by something of a depression from 

 the later rapidly widening whorls, which bear the character- 

 istic sculpture of spiral threads of three orders of strength, 

 with finely graven interspaces. Seen under the microscope 

 the primary threads appear very broad in contrast to the 

 secondaries, and the tertiaries are just visible in the inter- 

 spaces as fine ridges. There seems to 1 be no definite order 

 of arrangement of the three types of threads. At some points 

 they branch and anastomose much as do the corrugations in 



76 Sacco, Piemonte, pte. 8, p. 83, pi. 2, figs. 54-60, incl., 1891 . ' 

 "Idem., p. 85, pi. 2, fig. 55. 



