HNO in i' 



younir, ill .-hape :in<l plan of .-ruljil HIT, lut the Scales :i 

 and the central valves have only OIK- .-lit. ' 

 Carpentrr'- .!> -eripljnii an<l Bgtm dill! r SMI: 



specimens before me, described ahove. li' the Ian 



cally distinct it may he called /. Bttbcaric 



Siihgrnus \' . Ti: ACII YI>I:I:M<>\ ( 'arpenter, 1863. 



., Kep. l',rit. &MO. Adv. Sri. L863, p, 

 G. O. SABS, Moll. lie-. A ret. Norv. p. 1 i 

 IVmi. - - cinereus L.). Boreochiton S\i;- in ; 

 p. lit! (proposed for C. ruber Lowe and C. marmoreiu Fahr.;. 

 (liitoii, Lepidoplewrus, Leptochiton, Ischnochitnn and /,'>/>//>//" 

 !' various authors. 



Valves exactly like those of Ischnochifou, hoth diitside and 

 within: girdle having the scales very small, close and sim...tli. 

 (Jills from one-half to over three-fourths the length of the ; 



This irroup was originally proposed as a suhgenusof I.r]nnn-1, 

 and the following species referred to it: refij><>r<>.<n.-< Cp 

 stinctus Old., trifidus Cpr., dentiens Old., gothicus Cpr., h<irt> 

 ( 1 pi-., unltxUii Cpr., flectens Cpr. Carpenter subsequently stated 

 that Trac.hydermon was equivalent to Gray's second section of 

 Ischnoehiton, " ** scales of mantle minute, granule-like." (1*. /. 

 S. 1847, p. 127), this section containing only the species P. >//'//'/'/'- 

 ////x C. cinereus L. and Lowe. Still later, Carpenter raised the group 

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

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

 from Carpenter's original list; not from among the species si; 

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

 influenced Carpenter and Dall to place this group next t> the 

 Leptochitons, but there is probably no affinity indicated l>y this 

 character, Trachydermon being phy to genetically as far from 7. 

 ffi iton as Ischnoehiton is. 



The length of the gill-row is so variable that genera cann 

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

 itllms and ruber I have ascertained the gills to l>e median, 

 as they are said to be also in the tcrobieukUvs and // 

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

 which data are available, interstinrtii.-*. the irills 



extend from two-thirds to over three-fourths the length <t' th. 



