Mehriam.] 



Distribution of the Neocene Sea-urchins. 



H3 



of Clypeaster ? {Ecli.) Brewerianus, in the upper portion of the 

 Miocene of Contra Costa County, is the lowest. That of Echi- 

 narachnius, excentricus, in the Quaternary, perhaps including the 

 latest Pliocene, is the highest. Between these lie the zone of 

 Scutella interlineata, in the Merced series, immediately below the 

 Quaternary of Seven-mile Beach, and the beds with Astrodapsis 

 and Scutella {Clypeaster) Gabbi, above the recognized Miocene of 

 Contra Costa County. 



Both of these last-mentioned, intermediate horizons are gen- 

 erally referred to the Pliocene, though the age of both has been 

 variously determined by different investigators. It is thought that 

 the distribution of their characteristic sea-urchins and the asso- 

 ciated molluscan forms throws some light on the question of their 

 relative age. 



THE SAN PABLI ) FORMATION AND ITS GEOLOGIC RELATIONS. 



The series of strata characterized by the presence of Astrodapsis 

 and Scutella [Clypeaster) Gabbi may, with reference to its more 

 important features, be treated as a distinct formation, and will be 

 referred to in this paper as the San Pablo Formation. 



Faunal and Lithological Characters of the San Pablo. — At all 

 of the localities at which the San Pablo is known, it is char- 

 acterized faunally by the presence of a peculiar assemblage of 

 genera and species, in which Astrodapsis is the most abundant 

 and easily recognized form. The fauna is known so far by about 

 fifty species, of which nearly one-third are peculiar to these beds, 

 about one-fourth are known also from the Contra Costa County 

 Miocene, and one-sixth from the Merced. A little less than one- 

 half of the known forms are extinct. The number of species 

 made use of in this calculation is, unfortunately, not large, but 

 the proportions have been about the same in all of the collec- 

 tions made at different localities by Gabb, Turner, and the writer, 

 so that we may consider them as approximately indicating the 

 relations of the most vigorous part of the then existing fauna to 

 that of the nearest periods. 



The sea-urchins form the most prominent organic feature of 



