380 



University of California. 



[Vol. 3. 



Search has been made in this fauna for species like those of 

 the lowest Miocene of Contra Costa County. So far there have 

 been found an Agasoma very much like gravida, a Dosinia of 

 the type of maihewsoni and a Chione of the type of mathewsoni. 

 The Dosinia and Chione are not identical in form with the types 

 hut come near them. This fauna probably belongs to the 

 Agasoma zone of the lower Miocene, and were it not for certain 

 complications that arise it might be designated as such. For 

 the present it may be called the zone of Turritella hoffmani. 



The complications just mentioned are due to the fact that in 

 many of the regions in the southern part of the state, where the 

 sandstone phase of the Miocene is developed extensively below 

 the shale, the fauna containing T. hoffmani is not discovered, but 

 in its place there is found an abundance of Turritella ocoyana 

 along with T. variata. In most of the localities only a few 

 species have been found with those two forms, but enough has 

 been seen to make it appear that this is not exactly the same 

 zone as that of T. hoffmani. On the other hand, there is evidence 

 enough to show that the two horizons are not far removed from 

 each other. Near Bakersneld, in the beds from which the type 

 of T. ocoyana was obtained, an extensive fauna has been dis- 

 covered. Most prominent among the species are three forms 

 of Agasoma, the most common of which is near gravida. In 

 some respects it is intermediate between A. gravida of the lower 

 Miocene and A. sinuata of the upper Miocene. 



Both the zone of T. hoffmani and that of T. ocoyana appear 

 to belong close to the Agasoma zone of Contra Costa County. It 

 is also probable that the T. hoffmani zone is the older of the two, 

 as its fauna is made up very largely of extinct forms. That of 

 the T. ocoyana beds at Kern river contains a much larger number 

 of recent species, it is generally more modern in its appearance, 

 and it also shows the Agasoma group much more highly developed 

 and much more common than in the T. hoffmani beds. 



In the southern part of the state we probably have two fairly 

 distinct zones of the lower Miocene. The question naturally 

 arises; are we to consider the beds in Contra Costa County as the 

 equivalent of one or both of these divisions? This much may be 

 said, viz.: The fauna of the Agasoma beds of Contra Costa 



