48 CALLOCHITON. 



open on each side through a small rounded papilla in the vicinity 

 of the third or fourth branchia, counting forward, and between the 

 line of the gill row and the side of the foot. (Dall.~) 



Genus X. CALLOCHITON Gray, 1847. 



Callochiton GRAY, P. Z. S. 1847, pp. 126, 168 ; type Ch. Icevis. 

 SHUTTLEW Bern. Mittheil., Juni, 1853, p. 65. Callochiton GRAY, 

 Guide Syst. Dist. Moll. B. M. p. 181, 1856. Callochiton CARPEN- 

 TER MS. 1871, and of DALL, Proc. U. S. Nat. Mus. 1882, p. 286, 

 not Callochiton of Carpenter's earlier writings (P. Z. S. 1865, p. 

 $76, etc.), Callistochiton. 



Valves exposed ; insertion plates rising out of porous or spongy 

 eaves and cut into numerous teeth ; sutural plates connected or 

 continuous across the shallow jugal sinus. 



Two subgenera are recognized by Carpenter : 



Subgenus CALLOCHITON (restricted). 



Girdle covered with a smooth, compact layer of very small dia- 

 mond-shaped scales. 



Subgenus STEREOCHITON Cpr. 



Girdle leathery, more or less downy, frequently naked when the 

 delicate scales are rubbed off. 



Subgenus CALLOCHITON s. sir. 



The essential characters of the re&tricted genus Callochiton are 

 correctly stated by Carpenter as follows: (1) The insertion plates 

 are broken up into very numerous teeth rising out of spongy eaves; 

 (2) that these have a tendency to become propped outside; (3) that 

 the sinus is a mere wave in the otherwise united branches of the 

 sutural laminae ; (4) that the mantle is reticulated with what look 

 like diamond-shaped scales, which are only the points of long, flat- 

 tened, closely appressed corneous bodies, not found on any other 

 Chiton. For Gray's second group, in which the peculiar covering 

 is not seen, a subgenus should be created, unless indeed the species 

 may rank under Trachyradsia. For two species with a mantle 

 intermediate between Chsetopleura and Tonicella I have established 

 a subgenus Stereochiton. Of the whole series there are very few 

 species known. All of them are rare and display very little sculp- 

 ture on the valves. 



