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 f, mantle scales minute, granular ; ' but as the gill- 

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

 Trachydermon * * *." 



Of G. 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 CJiiton census. 



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

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

 Tonicise, 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 ToniceUa." 



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 



