n; \. in DERMOH, 



IT- 'in Tally in quinOUOoial pattern. liisrrlMii plah-~ -liort ami -harp, 

 having slits. Bavei solid or slightly porous. <iirdl- densely 

 clothed \vithvery minute rounded or elODgate papillft, 'I'y: 

 < pr. 



This group was formerly considered a subgenus of Ischnochiton 

 l>v me: luit :i critical review of the species, with the use of power 

 adequate to thoroughly reveal the structure of the girdle, cause 

 to reinstate it as a genus. It differs from Ischnochiton, even the 

 smallest species and specimens in the nature of the girdle cover- 

 ing: and the same is true of its relations with Chcetopleura, Callo- 

 chiton and Tonicia. The first of these three is also distinguished by 

 its peculiar sculpture ; the second by its continuous sutural lamina?. 

 Tniticin is the genus most allied to Trachydermon ; and it was no 

 doubt derived from Trachydermon at no remote time. 



The types selected by previous authors for this group are in no 

 case tenable. Carpenter's original list of Trachydermons comprised 

 reteporosus, inter stinctus, trifidus, dentiens, gothicus, hartwegii, nut- 

 tallii and flectens. Of these the first three are Ischnochitons ; 

 hartwegii and nuttallii belong to Cyanoplax ; leaving only dentiens, 

 gothicus and flectens available for the choice of a type. The last has 

 been selected. 



Within Trachydermon three sections may be distinguished, but 

 their differential characters are of little value. 



I. Trachydermon s. sir. (type flectens Cpr). Valves thin ; gills 

 extending forward t to f the length of the foot. 



II. Boreochiton Sars (type ruber L.). Valves variegated ; gills 

 median. Species, T. ruber, T.punicea, T. steinenii. 



III. Cyanoplax Pils. (type hartwegii Cpr.). Valves solid, thick ; 

 eaves wide, pitted ; gills as long as the foot. Species, T. hartwegii t 

 T. bipunctata. 



Besides these, a subgenus (Spongioradsia) has been created for 

 two divergent forms. 



The genus is one of great antiquity, being the least differentiated 

 of the Ischnochitonid(c. The girdle is unspecialized, being clothed 

 with minute bodies which cannot be called either scales or spines, 

 for they are of an intermediate character. See pi. 15, figs 

 (dentiens) ; 25 (ruber) ; and 37 (flectens). 



Thiele has proposed the " genus" Adriella for one of the typical 

 forms, founding it on a very slight difference in dentition, the value 



