415 CALIFORNIA ACADEMY OF SCIENCES [Pkoc. 4th Ser. 



It is of some interest in this connection to note that the 

 most abundant living species in the region whence practically 

 all the fossils came are probably Lepidochitona dentiens, Cyano- 

 plax hartwegii, Nuttallina fluxa, Ischnochiton conspicuus, 

 Ischnochiton pectinulatus, Mopalia muscosa, and Leptochiton 

 rugatus. 



Faunal Relations 



It is too early in the study and the available material still 

 far too scanty to permit much indulgence in the pastime of 

 generalizing with any great degree of safety, but a few points 

 which can be brought out with some measure of clearness by 

 a study of the accompanying table are perhaps worthy of brief 

 summarization. We are better protected than otherwise in 

 proceeding thus because the facts in evidence stand fairly in 

 corroboration of the major conclusions to which students of 

 the other animal groups have been impelled by working out 

 parallel data. 



A recapitulation of the table shows the total number of 

 species and subspecies for each of the principal formations to be 

 as follows: 



Specifically 

 determinable Total 



Oligocene 1 1 



Pliocene 2(3?) 3(4?) 



Pleistocene 28 33 



Lower San Pedro 19 



Upper San Pedro 11 



Uncertain 4 



Pliocene 



Leaving out the unique and extinct Oligocene species as 

 insufficient of itself to point conclusively one way or the other, 

 we find that the only certainly Pliocene species are Katharina 

 tunicata and Cryptochiton stelleri, both northern forms, as 

 would be expected by analogy with the remainder of the Plio- 

 cene fauna. These two species, however, happen both to be 

 of wide distribution, ranging from south-central California as 

 far north as Bering Sea, so do not help greatly in determin- 

 ing just how much colder than now the southern Californian 

 waters of that period probably were. 



