Clark: The San Lorenzo Series of Middle California 71 



to Weaver's Molopophorus lincolnensis zone. The following extracts 

 are taken from this paper : 



Our knowledge of the Oligocene of the Pacific Coast is very inadequate. In 

 order that we may study the Oligocene, its fauna must be first described. The 

 description of thirty-six new species from a fauna of forty-eight specifically 

 identifiable forms obtained from a single locality are given below. Better testi- 

 mony concerning our ignorance of the Oligocene could hardly be given when the 

 discovery of a new locality by two such good collectors and enthusiastic palaeon- 

 tologists as Mr. F. M. Anderson and Mr. Bruce Martin results in finding a fauna 

 which is seventy-five per cent new. 55 



No Tejon species occur in this fauna, yet its general cast is eocenic and some 

 of the species such as Exilia weaveri, Galeodea dalli, Neverita nomlandi, Triforis 

 martini, Solen lincolnensis, are apparently congeneric with forms found in the 

 Tejon Eocene of the Cowlitz River, Washington. Likewise no living forms are 

 contained in this fauna. 



The character of the sediments and the abundance of Hipponyx ornata, Hip- 

 ponyx arnoldi, Patella sub qua drat a, Crepidula sp. and Acmaea simplex, sessile 

 shore forms, mark this fauna as a strictly littoral one. In conclusion the fauna 

 appears to belong to a lower facies of the Molopophorus lincolnensis zone of 

 Weaver, and its distinctiveness is due in part to its strictly littoral character and 

 in part to having lived in a portion of Oligocene time older than that of the typical 

 Molopophorus lincolnensis zone. 



The presence of the genera Actaeon, Conus, Epitonium, Exilia, Fasciolaria, 

 Marginella, Serapis, Strepsidxira, Barbatia and Lima mark this fauna as sub- 

 tropical. This character is in accord with the assignment of this fauna to the 

 Molopophorus lincolnensis zone, the San Lorenzo of Arnold and Hannibal, who 

 inferentially recognized the tropical character of the Lower Oligocene. 56 



Arnold and Hannibal 's list from their Seattle horizon 5 "" is essentially the same. 

 The disappearance of many tropical genera, the introduction of several temperate 

 genera, are noteworthy temperate faunal conditions. That the Turritella porter- 

 ensis zone was tropical or semi-tropical is well attested by the occurrence of the 

 reef -building coral, Dendrophyllia hannibali Nomland and other tropical genera. 

 All the known facts considered, we may then conclude that the Molopophorus 

 lincolnensis and Turritella porterensis zones were deposited under tropical or 

 subtropical conditions and the Acila gettysburgensis zone, under temperate con- 

 ditions somewhat warmer than those of today in that latitude. What was the 

 reason for this faunal change? May we invoke the great god Diastrophism to 

 aid us in explanation? Probably a depression in the vicinity of the Bering 

 region of Alaska occurred at the beginning of the deposition of the Acila gettys- 

 burgensis zone and cold boreal waters of the Arctic sea brought with them a 

 boreal fauna some of whose members managed to establish themselves in Wash- 

 ington, in some eases even crowding out the native species. The known history 

 of Oligocene vertebrates gives some decided support to this hypothesis. Aecord- 



55 Diekerson, R. E., Climate and its influence upon the Oligocene faunas of 

 the Pacific Coast, with descriptions of some new species from the Molopophorus 

 lincolnensis zone, Proc. Calif. Acad. Sci., ser. 4, vol. 7, no. 6, p. 158, 1917. 



5« Op. cit., pp. 161-1(53, 165. 



570 The Seattle horizon of Arnold and Hannibal is the equivalent of the Acila 

 gettyburgensis zone of Weaver. 



