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



above and below, with scales, spines, seta?, or other horny or 

 calcareous appendages. When some of our fossiliferous rocks 

 have had their due of microscopic investigation, no doubt 

 chiton girdle scales will be found, perhaps sometimes even in 

 abundance. But up to the present there has been no recovery 

 of any remains beyond the valves of the shell. All other struc- 

 tures, being of but problematic value to the paleontologist, will 

 be dealt with only incidentally in this paper. Except where for 

 special reasons it becomes hardly avoidable, discussion of all 

 such will be left for papers dealing primarily with the recent 

 chitons, to which the student who wishes to push the matter 

 further may readily refer. 



The paleontologist, as sucJi, has to content himself with an 

 odd valve pried here and there from the rocks, rarely with any 

 evident remaining connection with any of the other valves 

 originally its fellows, so the systematic discussion in a work 

 of this scope necessarily must take purview of the situation and 

 concern itself chiefly with the description and identification of 

 such fragmental remains. 



The Shell of Chitons 



The valves of the chiton shell are not alike, nor are any two 

 of them absolutely so, though the more central ones are very 

 similar. To the casual observer they fall easily into three prin- 

 cipal categories: the anteriormost, or "head" valve, as it is 

 commonly called, the six intermediate or median valves, and 

 the poster iormost or "tail" valve. Perhaps these terms are not 

 strictly scientific either in etymology or application, but they are 

 the ones generally in use and the most convenient we have. 

 Each valve in articulating with its neighbors juts under the one 

 just in front and over the one just behind. Consequently the 

 head valve differs from all the others in being not only more or 

 less evenly crescentic, due to its terminal position, but without 

 accessory plates in front other than the marginal insertion plate 

 present in nearly all chitons except a part of the family Lepido- 

 pleuridce. All the other valves have a pair of sharp, plate-like 

 apophyses projecting on either side in front, the sutural lamina. 

 The tail valve is, however, not straight or angular behind, as 

 are usually the intermediate valves, but is evenly crescentic or 

 speciallv shaped in some other way. Its oldest or umbonal 



