260 CALLISTOCHITON. 



K< y to genera of Callistoplacince. 



a. Surface of valves having strong radial ribs ; girdle densely 



clothed with imbricating scales, CALLISTOCHITON. 



aa. Surface of valves granulated or pebbly ; girdle not densely 



imbricated with scales. 



b. Anterior valve with more than 7 slits ; sinus very spongy ; 



imicro posterior, NUTTALLINA. 



bb. Anterior valve with 5 slits; mucro subcentral, not posterior, 



CRASPEDOCHITON. ANGASIA. 

 aaa. Surface of valves wrinkled or ribbed ; girdle naked except for 



hairs or corneous spines. 



b. Valves having very strong radial ribs; girdle naked, with 



sutural tufts, CALLISTOPLAX. 



bb. Valves not very strongly sculptured ; girdle tough, with 



corneous curved spine-like processes, CERATOZONA. 



Genus XXIII. CALLISTOCHITON Carpenter, 1882. 



Callistochiton CPR., Table of Regular Chitons, 1873. DALL, Proc, 

 U. S. Nat. Mus. 1881, p. 283, 289, 290 (Jan.-Feb., 1882). Callo- 

 chiton (in part) H. & A. Adams, and of CARPENTER, P. Z. S. 1865, 

 p. 276, not of Gray. 



Valves conspicuously sculptured ; the insertion-plates short, 

 smooth or nearly so, festooned, being curved outward at the ribs 

 and slit there, thickened outside at the edges of the slits, the latter 

 corresponding in position to the ribs of the outer surface. Sinua 

 squared. Mucro median or post median, generally depressed. 

 Girdle poreless, densely clothed with minute striated or smooth 

 scales. Type, Chiton pukhellw Gray. 



This genus differs from Ischnochiton in the peculiar insertion-teeth, 

 which are curved into the ribs as if festooned, in the relation of the 

 slits to the external ribs, and in the tail valve, which is often 

 peculiarly humped. 



The species though not numerous, are widely distributed, Aus- 

 tralia, Red Sea, Japan and the Gulf of Mexico together furnishing 

 about half, the rest being from the western coast of the Americas, 

 from southern California to northern Chili. 



Nearly all of the species have the lateral areas strongly bicostate 

 and granose; the end valves have strong ribs, the number of which 



