314 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



mens. The less important details are very variable in di£ferent indi- 

 viduals. 



In taking leave of tbe Irregular Chitons, a few notes on exotic species 

 of this section may be properly incorporated. 



In Ghitonellus fasciatus, the representative of the most highly devel- 

 oped type of Chiton, the gill-rows are confined to the posterior quarter 

 of the foot, but the separate branchiie of which they are composed are 

 very large, twenty-six or eight in number, and rather long. There was 

 no well-marked crop, as in ordinary Chitons. The muzzle was inconspic- 

 uous, angulated at the posterior comers, with no veil. Mantle hardly 

 visible. There seemed to be two oviducts leading from a single ovary 

 (compounded of two ?) to small orifices, one on each side of the anus. 



In Cryptoconchus monticularis Quoy, which much recalls the northern 

 Katlwrina^ the girdle varied from black to light brown. A veil was 

 present, but narrow and simple, while the mantle-edge was hardly per- 

 ceptible. Gill-rows one-third as long as the foot, containing each about 

 eighteen branchige. Miizzle very transverse, with flaps at the posterior 

 comers. Ovisac single. 



CHITONES REGULARES. 

 LEPTOIDEA. 



Genus LEPTOCHITON Gray. 



Leptochiton Gray, P. Z. S. 1847, p. 127 ; Guide, p. 182, 1857. 



< Leptochiton H. & A. Adams, Gen. Eec. Moll, i, p. 473, 1854. — Chenu, Man. Conchyl. i, 



p. 381, 1859, etc. 



< Lepidopleurus Eisso (ex Leach MS.), 1826. — Sars, Moll. Reg. Arct. Norvegise, p. 110, 



1878. 



Lam. insertionis nullis; zona minutissime sabulosa; sinus Isevls; baud 

 laminatus; branchice breves. Type i. aseZtos Lowe. 



The diagnosis of Gray determines the genus, but he includes in the 

 examiiles cited C. alhus L., which is a Trachydermon. Two out of twenty- 

 five species cited by the brothers Adams are real Leptochitons j the 

 example cited by them as tyx^ical is not a LeptocJdtofi, neither is the 

 exami)le cited by Chenu. The other Leptoid genera are as follows : 



Hanleyia Gray, Guide, p. 186, 1857. 



Anterior valve with an nnslit insertion-plate ; other valves ■without even the 

 plates. H. (lebilis Gray. 



Hemiarthrum Carpenter, Bull. U. S. Nat. Mus. iii, p. 44, 187G. 



lusertion-iilates present on all the valves, but entire without slits. H. setttlosum 

 Cpr. 1. c. 



Deshayesiella Carpenter MS. 



LoricS, elongata; valv£B curvatse, antice tendentes; mucro planatus, zou4 spicu- 

 losii ; lam. insert. nuUis ; lam. sut. triangulares, extantibus. D. {L^tochiton) 

 curvatus Cpr. 



I 



