324 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



submediano, apicibus prominentibus ; ar. lat. inconspicuis ; tota super- 

 ficie qnincuncialiter minute reticulata ; intus, v. ant. 16, post. 11, centr. 

 2-lissata ; dent, parvis perspongiosis, late separatis ; subgrundis spon- 

 giosis, curtis ; sinu parvoj zona squamulis ininutis obsita. Lon. 6, Lat. 

 3 mm. 



Hab. — Kyska Harbor, Kyska Id. Constantine and Kiriloff' Harbors, 

 Amcliitka Island, and Nazan Bay, Atka, in the Western Aleutians, at 

 low- water mark, under stones on the beach, Dall ! Fifteen examples. 



This modest little species is of a dull livid purplish red, with an ashy 

 tinge, especially on the narrow girdle. Except for the well-marked 

 ridges of growth, it appears smooth, but possesses (like all Chitons) a 

 fine reticulation, only visible under a magnifier. The lateral areas are 

 not distinct, the back is very much rounded, and the valves well hooked 

 in the median line. The substance of the valves from within appears 

 remarkably spongy, as if rotten, or even like vesicular pumice, espe- 

 cially under the eaves. The anterior slits are marked by radiating lines 

 of holes, though the teeth between them can hardly be made out. The 

 posterior valve, however, has not this aid to counting, and in the general 

 sponginess it is almost impossible to say how many teeth or denticles 

 exist. It bears no marked resemblance to any other species of the re- 

 gion. 



Genus TONICELLA Cpr. « 



Tonicella Cpr. Bull. Essex Inst, v, p. L54, 1873. Type T. marmorea Faur. 

 Tonicia sp. Adams, Gray, Cpr. and Others. 



< Boreochiton G. O. Sarn, Moll. Reg. Arc. Norv. p. 110, Juuc, 1878. 



Valvae, mucro, laminse et sinus plerumque ut in Ischnochitone; zona 

 ut in Tonicia^ coriacea, lasvis, sen snblaevis: brancMffi mediae. 



The genus Tonicia Adams and Gray, to which the species of Tonicella 

 have often been referred, bias pectinated insertion-plates and ambient 

 gills like the typical Chitons, while Tonicella has sharp plates and short 

 jo \v.v, of gills. The two groups also differ in their dentition. The major 

 lateral of Tonicella is strongly tridentate ; in Tonicia the cusp of the 

 major lateral is scoop-shaped, rounded, with a plain edge, and the radula 

 recalls that of Chiton (typical) and Corephium. Prof. Sars appears to 

 have been unaware of Dr. Carpenter's publication on the New England 

 Chitons. 



Tonicella marmorea. 



Chiton marmoreus Fabricius, Fauu. Gronl. 420, 1780. — Midd. Mai. Ross, i, p. 103, 



1847; Sib. Reise, 182, 1851,-^Forbes & Hanley, Brit. Moll, ii, p. 414, pi. 



lviii, f. 2, pi. lix, f. 4, 1853.— Jeffreys, Brit. Conch, iii, p. 227, 18G5, v, p. 



199, pL lvi, f. 7, 18o9. 

 Chiton ruber Sponsor, Bkrift. Nat. Sclsk. iv, p. 92, 1797.— Loven, Ind. Moll. 



Scaud. p. 28, 1840 ; no1 of Linne\ 

 Chiton lecvigatus Fleming, Edin. Encycl. p. 113, t. vii; Brit. An. p. 290, 1828. — 



Reeve, Conch. Icon. Chiton, pi. 27, f. 179, 1847. 

 f Chiton punctatw Strom (Jeffreys)?, Acta Nidr. iii, p. 433, t. vi, f. 14. 



