Vol. VII] BERRY— WEST AMERICAN CHITONS 245 



2, On the Genus Trachydermon 



In the preceding note I have briefly mentioned the species 

 Trachydermon flectens Carpenter and indicated my behef that 

 the genus should be interpreted in an altogether different way 

 than has heretofore been recognized. This is entirely due to 

 the fact, brought about quite incidentally by a strict adherence 

 to the rules of the International Code of Zoological Nomen- 

 clature, that because of its subsequent selection for that rank by 

 Pilsbry, T. flectens must be recognized as the type of Carpen- 

 ter's genus. This exquisitely beautiful little chiton has been 

 known for many years and is frequently met with in collections, 

 yet, if I am correct in my identification of such specimens as I 

 have seen with Carpenter's type, it has been one of our most 

 misunderstood species. While its reference to the Mopaliidse 

 will no doubt occasion general surprise and will probably not be 

 allowed to pass unquestioned, it seems to me clear that T. flec- 

 tens has little in common with any of the numerous other 

 species commonly ranked with it, and which latter I fear Car- 

 penter himself really had most in mind at the time he founded 

 his genus. It does not even belong in the subfamily to which 

 in recent years it has given its name. Fortunately the recent 

 rehabilitation of Gray's Lepidochitona by Iredale ( :14, p. 127) 

 -has already precipitated the nomenclatorial changes which 

 would otherwise have been the inevitable result of removing 

 the type species of Trachydermon to another family. 



The accumulation of evidence which has finally led me to the 

 adoption of the course outlined had its beginning in the almost 

 absolute parallel which is to be found between Pilsbry's de- 

 scription and figures of T. flectens in the concluding part of 

 the "Manual" ('93, p. 64, pi. 15, figs. 34-37), and his later 

 description of Mopalia heathii ('98, p. 288). By tabulating in 

 parallel columns the essential items, not only of these two de- 

 scriptions, but of Carpenter's own brief diagnosis (Pilsbry, '92, 

 p. 75) and the present writer's more recent redescription of 

 heathii (Berry, :11, p. 490, text figs. 4-7, pi. 40, figs. 1-3, 7), 

 the strength of the evidence can be more readily appreciated 

 and the reader then left to draw his own conclusions. Such 

 discrepancies as then appear are thus thrown into relief for 

 consideration later. 



