320 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



Lophyrussp. G. O. Sars, 1. c. p. 114, not of Poli. 

 <Boreochiton G. O. Sars, 1. c. p. 115. 

 Leptochiton sp. auct. 



Char. — Laminae inserentes acutse, heves; valvse extus et intus Iseli- 

 nochitoni exacte simulans; zona non porifera, squainulis minutissiinis 

 l^vibus confertissime granulata; branchiae breves. 



This name was originally proposed as a subgenus of Isclmochiton to 

 include Gray's second section, " mantle scales minute, granular" (P. Z. S. 

 1847, p. 147; Guide, p. 182, 1857). In 



all other conchological characters, the fVt^ P^iffT 

 group accords with that genus, but the 

 animal differs in having the gills either 

 entirely posterior or reaching forward 

 from the tail only to about the middle 

 of the foot, while in Isclmochiton and 



_ ' . Pig. E.— Teeth of Trachyderm on cinereus 



Chiton they travel to its anterior ex- Lowe; after Loven. 



tremity. These characters indicate a transition between the Ischnoid 

 and Leptoid Chitons by means of Trachydermon and Tonicella, Guilding 

 called the radulaof Chitons "Trachy derma"; but as the name has not 

 been adopted, no inconvenience is likely to ensue. (Cpr. MSS.) 



The genus is chiefly northern in its distribution. Chiton marginatus 

 of authors (Pennant's species being indeterminable) and C. cincreus (Linn.) 

 Lowe, are identical, according to Dr. Carpenter, the best authority on 

 the subject, as well as Hauley and others. The u Lepidopleurus v cincrcus 

 of Sars is not the Linnean species, which is the type of Trachydermon, 

 but a Leptochiton. His Craspedochilus marginatus (whether the Chiton 

 marginatus of Pennant or not) is a Trachydermon, and not improbably 

 the true cinereus of Linne, which has been recognized, not from the 

 insufficient description in the Syst. Naturae, but from his typical speci- 

 mens, through the invaluable labors of Mr. Hanley. 



Trachydermon ruber. 



Chitoh ruber Linn. S. X. ed. xii, p. 1107, 1766. — Lowe, Zool. Jonrn. ii, p. 101, 

 pL 5, f. 2, 1825.— Gould, Inv. Mass. p. 149, f. 24, 1841.— Forbes & Hanley, 

 Brit. Moll, ii, p. 399, pi. lix, f. 6; AA, f. 6, 1853.— Hanley, Shells of Lin. 

 p. 17, 1855.— Sowcrby, Conch. 111. Chiton, f. 103-4, 1839.— Reeve, Conch. 

 Icon. Mon. Chiton, pi. 25, f. 175, 1847. — Jeffreys, Brit. Conch, iii, p. 224, 

 1865; v, p. 199, pi. lvi, f. 4, 1869.— Binncy's Gould's Inv. Mass. p. 260, f. 

 523, 1870. 



Chiton cinereus O. Fabr. Faun. Gronl. p. 423, 1780; not of authors, nor of Linn. ; 



lb. Dillwyn, Cat. Rec. Sh. p. 12, 1817. 

 Chiton minimus Spongier, Shrift. Nat. Selsk. iv, 1, 1797, lido Loven, not of 



Grnelin and Chemnitz. 

 Chiton Icevis Lov6n, Ind. Moll. Lit. Scand. p. 28, 1846; not of Montague, 



Forbes and Hanley, etc. 

 Chiton leevis Pennant (probably), Brit. Zool. ed. iv, vol. iv, p. 72, pi. 36, f. 3, 



1777 (bad). 



Chiton latits Leach, Moll. Brit. p. 231, 1852, Doc, fide Jeffreys; not of Lowe, 

 1825. 



