C^C 



PROCEEDINGS 



OF THE 



CALIFORNIA ACADEMY OF SCIENCES 



Fourth Series 



Vol. XI, No. 18, pp. 399-526, pis. 1-16, text figs. 1-11, May 16, 1922 



XVIII 



FOSSIL CHITONS OF WESTERN NORTH AMERICA. 



Ss 



S. STILLMAN BERRfe^ 



REDLANDS, CALIFORNIA. 



Introduction 



v A/ ^ 



Begun primarily as a simple record of the occurrence of 

 chitons in the later Tertiary and Quaternary deposits of south- 

 ern California, the unexpected accretion of altogether unusual 

 amounts of material from ever-increasing sources has neces- 

 sarily impelled a widening in scope of the work in hand until 

 it is now frankly offered as a monographic survey of the known 

 fossil Polyplacophora of western North America. 



Chitons are not generally considered as having much im- 

 portance as fossils, and, taking the world as a field, surprisingly 

 few fossil species or even specimens have been brought to 

 light and recorded. Yet there are reasons for believing that 

 the members of this group, when they do occur, have an in- 

 trinsic value as paleontologic criteria rather above that of most 

 groups of Mollusca. At any rate, it is not altogether well to 

 neglect them. The chief reasons for believing that the chitons 

 furnish relatively conservative, and therefore correspondingly 

 dependable, indices are: firstly, that on the Pacific Coast of 

 North America this group is by no means of rare occurrence in 

 the later fossiliferous horizons, as has been more or less im- 

 plicitly assumed in the past, but attains a development paral- 

 leled only by the remarkable amplification of the entire class in 



May 16, 1922 



