TRACHYDERMON. 63 



generally in quiucuncial pattern. Insertion plates short and sharp, 

 having slits. Eaves solid or slightly porous. Girdle densely 

 clothed with very minute rounded or elongate papillae. Type T. 

 flectens Cpr. 



This group was formerly considered a subgenus of Isehnochiton 

 by me ; but a critical review of the species, with the use of power 

 adequate to thoroughly reveal the structure of the girdle, causes me 

 to reinstate it as a genus. It differs from Isehnochiton, — 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 laminoe. 

 Tonicia is the genus most allied to Trachijdermon ; 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, interstinctus, trifidvs, dentiens, gothicus, hartivegii, nut- 

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

 hartivegii 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. str. (type flectens Cpr). Valves thin ; gills 

 extending forward t to I the length of the foot. 



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

 median. Species, T. ruber, Tpunicea, T steinenii. 



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

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

 T. bipimctata. 



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 Ischnochitonidce. 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. 26 

 (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 



