ISCHNOCHITON. 67 



young, in shape and plan of sculpture, but the scales are not chaffy 

 and the central valves have only one slit. (Cpr.~) 



Carpenter's description and figure differ somewhat from the 

 specimens before me, described above. If the latter proves specifi- 

 cally distinct it may be called 1. subcariosus. 



Subgenus V. TRACHYDERMON Carpenter, 1863. 



Trachydermon CPR., Rep. Brit. Asso. Adv. Sci. 1863, p. 649. 

 Crasvedochilus G. O. .SARS, Moll. Reg. Arct. Norv. p. 1 14 (for C. 

 marginatus Penn. = cinereus L.). Boreochiton SARS (in part), L c., 

 p. 116 (proposed for C. ruber Lowe and C. marmoreus Fabr.). 

 Chiton, Lepidopleurus, Leptochiton, Isehnochiton and Lophyrus sp., 

 of various authors. 



Valves exactly like those of Isehnochiton , both outside and 

 within ; girdle having the scales very small, close and smooth, 

 Gills from one-half to over three-fourths the length of the foot. 



This group was originally proposed as a subgenus of Isehnochiton, 

 and the following species referred to it : retiporosus Cpr., inter- 

 stinctus Gld., trifidus Cpr., dentiens Gld., gothicus Cpr., hartwegii 

 Cpr., nuttallii Cpr., flectens Cpr. Carpenter subsequently stated 

 that Trachydermon was equivalent to Gray's second section of 

 Isehnochiton, " ** scales of mantle minute, granule-like." (P. Z. 

 S. 1847, p. 127), this section containing only the species P. margin- 

 atus= C. cinereus L. and Lowe. Still later, Carpenter raised the group 

 to the rank of a genus, adding to his diagnosis the words " gills short." 

 It is evident that a type species for the group should be selected 

 from Carpenter's original list ; not from among the species subse- 

 quently referred to the group by him. The shortness of the gills 

 influenced Carpenter and Dall to place this group next to the 

 Leptochitons, but there is probably no affinity indicated by this 

 character, Trachydermon being phylo genetically as far from Lepto- 

 chiton as Isehnochiton is. 



The length of the gill-row is so variable that genera cannot be 

 founded upon that character in the present family. In the species 

 albus and ruber I have ascertained the gills to be median, 

 as they are said to be also in the scrobiculatus and lividus 

 of Midd. In the other species which I have examined, or upon 

 which data are available, inter stinctus, cinereus, dentiens, the gills 

 extend from two-thirds to over three-fourths the length of the foot. 



