THE NAUTILUS. 63 



forms are well separated the series yet approach very near if they do 

 not overlap. 



In spite of this the two are to-day almost universally placed by 

 American and British malacologists in two distinct genera — genera 

 which, in Pilsbry's monograph [Pilsbry '92] are rather widely sepa- 

 rated. 



I propose very briefly to review their recent generic history. 



Both were included in Gray's genus Ischnochiton, from which Car- 

 penter ['63] split off Trachydermon as a sub-genus, naming no type 

 though his own retiporosus was the first of the species referred to it. 

 Neither ruber nor marmoreus was mentioned. He later declared 

 it corresponded to the " second section " of Gray's Ischnochiton 

 " * * * scales of mantle minute, granule-like," and raised it to the 

 rank of a genus. 



In 1873 [Carpenter '73] he published brief and informal, but im- 

 portant, notes of his observations on the Chitons of our New Eng- 

 land Coast made during work with our Fish Commission expeditions. 

 Of C. ruber he said : " It belongs to Gray's genus Ischnochiton, 

 * * * ' section |, mantle scales minute, granular ; ' but as the gill- 

 rows are short * * * it is necessary to establish a fresh genus, 

 Trachydermon * * *." 



Of C. albus — now by far the commonest species all along our shore, 

 outnumbering ruber and marmorea combined a great many times 

 over and scarcely to be missed by any dredger — he said : " I twice 

 captured a live specimen ; but each time it eluded the aftersearch. 

 I do not doubt that this is also a Trachydermon * * *." While not 

 germane to the present inquiry this is interestingly suggestive of 

 changes in the Chiton census. 



Of C. marmoreus, he said : [it] " is Tonicia of H. Adams and 

 Gray, simply because the girdle is smooth. The true southern 

 Toniciee, however, have pectinated insertion-plates and ambient gills, 

 like the typical Chitons ; while the northern species so-called have 

 sharp plates and short gills. They differ, in fact, from Trachy- 

 dermon simply in the girdle being destitute of the minute scales. I 

 distinguish the group as Tonicella." 



This is the origin of the latter genus, which we now know is far 

 removed from true Tonicia. 



Tonicella, then, was originally nothing but a scaleless-girdled 

 Trachydermon, with marmorea as its type-species. I have looked in 



