76 University of California Publications in Geology [VOL. 8 



FAUNA OF THE TROCHOCYATHUS ZITTELI ZONE NORTH OP MOUNT DIABLO (Continued} 



Loc. Loc. Loc. Loc. Loc. Loc. Loc. Loc. Loc. 

 1540 1580 1746 1747 1656 1695 1586 1592 1558 



Surcula, sp x 



Siphonalia(?) lineata Stanton x 



Turritella pachecoensis Stanton x x x 



Turritella clarki, n. sp. . x 



Tritonium, sp., a x 



Turris, sp x x 



Turris louderbacki, n. sp x 



Urosyca caudata Gabb x x x x 



Xenophora zitteli Weaver x .... x x 



Trochocyathus zitteli (Merriam), Lima haseltinei, n. sp., Crassatel- 

 lites stewart villensis, n. sp., Anchura gabbi, n. sp., Ficopsis, sp., Nep- 

 tunea cretacea Gabb, Surcula fairbanksi, n. sp., and Turritella clarki, 

 n. sp., are some of the forms which have not been found in the 

 Meretrix dalli zone or in the Solen stantoni zone at the type locality. 

 The principal difference between this fauna and that of the lower beds 

 is in the ratio of the gastropods to the pelecypods, viz., 3:2 in the 

 Trochocyathus zitteli zone, 3:4 in the Meretrix dalli beds. This 

 difference is still greater when all the gastropods and pelecypods are 

 enumerated. At least two factors figure in the cause of these differ- 

 ences bathymetric conditions and difference in age of the two zones. 

 The bathymetric conditions of the lower beds have already been 

 described. The genus Trochocyathus, according to the "Challenger" 

 Report 19 ranges from a depth of 100 to 750 fathoms. Glauconite is 

 abundantly formed at a depth of a hundred fathoms or more in the 

 ocean of the present. A study of the gastropods brings us to the 

 same conclusion, viz., that the Trochocyathus zitteli beds were 

 deposited in at least one hundred fathoms of water. The age differ- 

 ence between the faunas of the two zones is obvious, the one being 

 stratigraphically above the other. Just how much control these two 

 factors exert is an exceedingly difficult matter to determine. The 

 absence of Trochocyathus in the lower beds may be explained upon 

 a basis of depth relations, but why certain forms such as Cardita 

 veneriformis Gabb, Leda milleri, n. sp., Meretrix dalli, n. sp. Tritonium 

 martinezensis, n. sp., and Cidaris, sp., a, should not appear in the 

 Trochocyathus zitteli zone cannot be explained upon the same basis, 

 for apparently some of these forms disappeared before the upper 



i Moseley, Challenger Report, Zoology, vol. 2, part 7, pp. 132-133, 1881 



