Family LEPIDOPLEURID^E. 



Leptoidea CARPENTER. Leptochitonidce DALL, ' Blake' Gastrop. 

 p. 414, 1889. Genus Holochiton FISCHER, Manuel de Conchy 1. p. 

 877, 1885. 



Chitons in which the head and tail valves are similarly articu- 

 lated, and having the insertion plates either obsolete, or if present, 

 without slits. Girdle finely scaly or bristly ; gills short, posterior. 



This family is readily known by the entire absence of insertion- 

 plates, or the simple, unslit character of them when present. The 

 living species are few in number, and mainly either northern in dis- 

 tribution, or living at considerable depths. All of the Paleozoic 

 Chitons yet known belong to this family, and this fact, together with 

 the weak, imperfect articulation of the valves, causes us to regard 

 the Leptoids as the most primitive of the existing groups. 



Synopsis of Genera. 



A. Valves entirely exposed, r.ot concealed by the girdle. 

 B. Girdle without pores. 



Genus I. LEPIDOPLEURUS Risso, 1826. 



Insertion plates absent. Girdle with minute, gravelly, smooth or 

 striated scales, usually with a marginal fringe of longer scales. 

 Type, L. cajetanus Poli. 



Section Deshayesiella Carpenter, 1878. Girdle having delicate 

 spines and chaffy scales. Valves curved and beaked; sutural 

 plates and sinus as in Leptochiton. Type, L. curvatus Cpr. 



< ienus II. HANLEYA Gray, 1857. 



Front valve having an insertion plate which is grooved outside 

 but without slits. Intermediate and posterior valves without inser- 

 tion plates. ( lirdle finely spiculose, without pores. Type, If. han- 



U-ijl liean. 



J3B. Girdle with pores at the sutures. 



<iem.s III. HEMIARTHRUM Carpenter, 1876. 



Head and tail valves having smooth, unslit insertion plates, the 

 intermediate valves without insertion plates. Sinus broad and 



