314 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



mens. The less important details are very variable in different indi- 

 viduals. 



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

 of this section may be properly incorporated. 



In Chitonellus 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 branchiae 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 corners, 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 Cryptoconclius monticularis Quoy, which much recalls the northern 

 Katkerina, 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 branchiae. Muzzle very transverse, with flaps at the posterior 

 corners. Ovisac single. 



CHITONES HEGULARES. 

 LEPTOIDEA. 



Genus LEPTOCHITON Gray. 



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



< Leptochiton H. & A. Adams, Gen. Rec. Moll, i, p. 473, 1854. — Chenu, Man. Conchy], i, 



p. 381, 1859, etc. 



< Lcpidopleiwm Risso (ex Leach MS.), 1826. — Sars, Moll. Reg. Arct. Norvegiae, p. 110, 



1878. 



Lam. insertionis nullis; zona minutissime sabulosa; sinus laevisj hand 

 laminatus; branchiae breves. Type L. asellus Lowe. 



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

 examples cited C. albus L., which is a Tracliydermon. Two out of twenty - 

 li ve species cited by the brothers Adams are real Leptochitons ; the 

 example cited by them as typical is not a Leptochiton, neither is the 

 example cited by Chenu. The other Leptoid genera are as follows : 



Hanleyia Gray, Guide, p. 180, 1657. 



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

 plates. II. debilis Gray. 



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



Insertion-plates present on all the valves, but entire without slits. H. sctuJosum 

 Cpr. 1. c. 



Deshayesiella Carpenter MS. 



Lorica elongata; valvse curvatse, antico tendentes; mucro planatus, zona spicu- 

 losa; lam. insert, nullis ; lam. sut. triangulares, extantibus. D. {Leptochiton) 

 curvatus Cpr. 



