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



side slopes as the pleural tracts. In Acanthochitona and some 

 other forms, the boundary between the jugal and pleural tracts 

 is better marked than that of the lateral areas, and the entire 

 region of the slopes is called the latero- pleural areas. The teg- 

 mentum of the median valves sometimes shows an angular 

 projection in the median line behind, the beak, and more rarely 

 a similar forward projection between the sutural laminae in 

 front. This, when present, is known as the false beak. 



The tegmentum of the head valve is not so divisible into 

 areas. Its sculpture is almost uniformly developed on the same 

 general plan as that of the lateral areas, and, as with them, is 

 primarily radial. The articulamentum of this valve is of course 

 entirely made up of insertion plate tissue. The tail valve, on 

 the other hand, has sutural laminae similar to those of the 

 median valves and its tegmentum is typically divided into two 

 regions, a central area in front of the mucro, and a posterior 

 area behind it. The central area coincides in significance and 

 essential plan of sculpture with the central areas of the median 

 valves. The posterior area is homologous with the lateral areas 

 and is usually similar to them in sculpture, though it develops 

 peculiarities of its own now and then. It is sometimes con- 

 venient to refer to the tegmental surface of the head valve and 

 the posterior area of the tail valve together as the terminal 

 areas, as they are frequently so similar in sculpture. The articu- 

 lamentum, and sometimes the tegmentum also, of the tail valve 

 is sometimes emarginated in the median line behind to form 

 a posterior sinus. 



SYSTEMATIC ACCOUNT 



General Remarks 



The general purport of the systematic portion of this paper 

 is to give a catalogue of the known species of fossil chitons of 

 western North America, systematically arranged, and in con- 

 nection with each species to present 



1. A brief summary of the synonymy, with those refer- 

 ences to the literature as seem of special importance to the 

 paleontologist, especially such as are accompanied by useful 

 figures ; 



