420 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



FORMATIONAL INTER-RELATIONSHIPS OF CHITON FAUNA 



Oligocene Pliocene Lower Upper Recent 



San Pedro San Pedro 



Oligocene 1 



Pliocene 3 



Pleistocene: 



Lower San Pedro. . 1 19 



Upper San Pedro.. 2 5 11 



Recent 3 18 9 110+ 



A conspicuous fact brought out in several of the tables is 

 the small number of species appearing in all formations which 

 are extinct or not yet known to occur living. The single Oligo- 

 cene species is unique and, as would be anticipated, is prob- 

 ably extinct. But with one exception all the Pliocene and 

 Lower Pleistocene forms which have been specifically de- 

 termined are also of recent occurrence. This is likewise true 

 of most of the fauna from the Upper (San Pedro) Pleistocene, 

 but two species here described as new from the rocks of this 

 period have not yet been discovered in the recent state. 



The species peculiar to each formation are : 



Number of Per cent, 



species of total 



Oligocene 1 100 



Pliocene 1* $$ 



Pleistocene: 



Lower San Pedro 13** 68.4 



Upper San Pedro 5*** 45.5 



*all occur also in living fauna. 



**all but one also known to occur in living fauna. 



***all but two also known to occur in living fauna. 



No doubt these figures will be very greatly modified by the 

 results of later investigation, but to some degree are probably 

 dependable. An analysis in detail would probably show the 

 more characteristic species of the respective formations to be 

 somewhat as follows, all but Mopalia acuta being, so far as is 

 known, confined to the formation under which they are listed : 



Oligocene : Oligochiton lioplax 



Pliocene: Katharine tunicata 



Pleistocene — 



Lower San Pedro Tonicella lineata 



Cyanoplax hartwegii 



fackenthallcc 

 Nuttallina calif ornica 

 Mopalia lignosa 

 " ciliata 



