CALLISTOCHITON. 261 



is less valuable as a specific character than one would at first suppose, 

 on account of their tendency to increase by splitting. When a rib 

 splits, an additional slit appears in the insertion-plate ; so that this 

 character also is largely vitiated. 



The best characters for distinguishing species are the sculpture of 

 the median portion of the central areas, and the contour of the tail 

 valve ; the profile of the latter being especially characteristic and 

 constant. 



The following notes from Carpenter's MS. apply more especially 

 to the more typical forms : " It is quite possible that in their earlier 

 stages these curious shells resemble Ischnochiton ; but as they pro- 

 ceed toward maturity, while they spread naturally in the seven ante- 

 rior valves, the posterior is simply raised a tier higher ; consequently, 

 in the adult, the posterior part of the tail plate resembles a closed 

 fist outside; and within the teeth and eaves are very broad and 

 blunt. There is an approach toward the throwing forward observed 

 in Acanthopleura. In Ischnochiton, whatever be the external sculpt- 

 ure, the row of teeth follow the same oval line, and the incisions do 

 not correspond (except by accident) with the external ribs ; whereas 

 in Callistochiton the tooth line is elegantly scalloped in and out 

 of the hollows of the ribs. 



" In general the incisions are in the centre of each rib, and the 

 plates are there propped outside, as in Callochiton ; but sometimes a 

 separate keystone (as it were) is let into some or all of the arches on 

 the terminal valves. The genus appears to culminate on the Cali- 

 fornia coast, and reaches its greatest development in C. palmulatus ; 

 in which the posterior valve displays the further peculiarity of each 

 tooth being broken up into a number of fingers, each fluted, and so 

 arranged that the inner margins form a semicircle while the outer 

 present the scalloped curve of the genus. The valves are peculiarly 

 solid and are easily detached from the thin and narrow zone. The 

 sinus has always a lamina, which is generally marked off by slits from 

 the sutural laminae, but is scarcely ever dentate. Two of the species 

 have minute, smooth scales." 



Key to species of Callistochiton. 



One species, C. heterodon Pils., has pectinated teeth like a typical 

 Chiton, and it is placed in this genus with much doubt ; the others 

 may be tabulated thus : 



