PALEONTOLOGY ZORRITOS FORMATION 13 



so far collected occur is far too resistant to permit the 

 extraction by any known means of specimens at all suscep- 

 tible to injury through moderate violence; the sands are 

 firmly cemented by calcium carbonate, and attempts at 

 removal of matrix by loosening it through chemical destruc- 

 tion of the cement fail because the shell material is of the 

 same composition, and is destroyed along with the matrix. 

 Loose matrix on some of the specimens in the Nelson 

 collection indicates that there are probably some localities 

 at which the sands have not been cemented, and with the 

 discovery of such places a large fauna may be revealed to 

 the paleontologist. 



II BIOLOGIC ANALYSIS. 



The present study has made known 101 species and 

 varieties of mollusks, of which 44 are gastropods and 57 

 pelecypods. . Two of the species are indeterminate and 64 

 are new. 31 occur in the Upper Zorritos formation, 15 in 

 the Variegated, 43 in the Lower Zorritos, and 20 from 

 Nelson's collection do not occur in the material collected by 

 Singewald, and are thus not stratigraphically located beyond 

 the high probability that they came from the Zorritos for- 

 mation. 



The gastropods are somewhat widely distributed through 

 the biologic scheme, most of the 18 genera being represented 

 by one or two species, and only two genera, Conus and 

 Turrit ella, being represented by more. In the case of the 

 Cones this is not surprising, since the normal tropical fauna 

 contains a heavy representation of that genus. The Tur- 

 ritellas, however, are not ordinarily diversified in the tropics 

 to the extent found in the Zorritos fauna, and the seventeen 

 species and varieties here presented seem entirely out of 

 proportion with the rest of the fauna. Accidents of collecting 

 might explain the relatively more complete assemblage of 

 the genus, and thus relieve the disproportionate aspect of the 

 group, but granted that, the number is still larger than one 



