14 



University of California Publications in Geology 



[Vol. 9 



which contain characteristic species of the Agasoma gravidum beds, 

 are found in the conglomerates at the base of the Area montereyana 

 beds. At some localities these boulders are so abundant that one, in 

 collecting from the matrix of the conglomerate, must be careful not 

 to gather a mixture of the upper and lower faunas. The writer has 

 collected ten or more species of the Agasoma gravidum fauna from 

 boulders in this basal conglomerate and undoubtedly a much larger 

 number could be obtained. Many boulders are quite angular and 

 were over a foot in diameter and apparently they could not have been 

 transported far. It seems possible therefore that they were derived 

 from cliffs in the vicinity of this beach during Monterey time. 



FAUNAL KELATIONSHIP OF AGASOMA GEAVIDUM ZONE TO AECA 



The best evidence for an important hiatus between the Agasoma 

 gravidum beds and the Area montereyana beds is palaeontological. 

 At present ninety molluscan species are known from the Agasoma 

 gravidum beds ; of these thirty-one, or over 40 per cent, of the deter- 

 minable species are new. Over sixty determinable species are known 

 from the Area montereyana zone of Contra Costa County ; of these 

 only twelve have been found in the Agasoma gravidum zone. None 

 of these twelve species common to the two zones are forms that are 

 generally considered to be good horizon determiners. Fifty-five of 

 the determinable species of the Agasoma gravidum zone have not been 

 found in the Area montereyana zone, and there is nearly an equal 

 number in the upper of the two zones which have not been found in 

 the lower. 



LIST OF SPECIES FROM AGASOMA GRAVIDUM ZONE OF CONTRA COSTA 

 COUNTY, CALIFORNIA 



Pelecypoda Leda, sp.? 



Area, n. sp., A. Macrocallista, n. sp. 



MONTEREYANA ZONE 



Area, n. sp., B. 

 Acila, n. sp. 

 Amiantis, n. sp. 

 ( Jardinm lorenzannm Arnold 

 Cardium vaqueroesensis Arnold 

 Cardiuni, n. sp. 

 Chione, n. sp., A. 

 Chione, n. sp., B. 

 Chione, n. sp., C. 

 Diplodonta, n. sp. 

 Donax, sp. indt. 

 Dosinia whitneyi (Gabb) 

 Leda, n. sp. 



Macrocallista cf. vespertina 



(Conrad) 

 Macrocallista? mathewsonii 



(Gabb) 

 Mya, n. sp. 



Mytilus mathewsonii Gabb 

 Mytilus, sp.? 

 Nueula, n. sp., A. 

 Nucula, n. sp., B. 

 Panope estrellana, Conrad 

 Periploma, sp.? 

 Pecten, n. sp. 

 Peeten peckhami Conrad 



