DESCRIPTIONS OF GENERA AND SPECIES. 



Order polyplacophora. 



Section I.— CHITONES REGULARES. Regular Chitons. 

 Head and tail plates of similar character. 



A.— Leptoidea. 



Insertion plates obsolete, or, if present, unslit. 



Leptochiton. — Insertion plates absent ; sinus simple, smooth, deep, not laminated ; 

 girdle with minute gravelly, smooth or striated scales ; gills, short, posterior. [Lepto- 

 chiton cancellatus, Sow.) 



Deshayesiella. — Shell elongated ; valves curved, pointing forwards ; umbo flattened ; 

 insertion plates absent ; sinus simple, broad ; sutural laminae triangular ; girdle with 

 spicules and chaffy scales. (Deshayesiella curvatus, A. Ad.) 



Hanleyia. — Insertion plate only in anterior valve, rugose but non-denticulate ; eaves 

 minute ; sinus simple, broad, shallow ; girdle with hairs or fine spines, lateral tufts small. 

 (Hanleyia debilis, Gray.) 



[ Hemiarthrum. — Insertion plates only in anterior and posterior valves; smooth unslit 

 laminae ; minute eaves ; broad spongy sinus ; girdle solid, downy, poriferous, lateral tufts 

 small ; gills short. (Hemiarthrum setulosum, Carp.) 



Microplax. — Insertion plates present in all the valves ; smooth unslit laminae ; no 

 eaves ; sinus obsolete ; girdle, thin, horny, finely grauulous. (Microplax grayi, Ad. and 

 Aug.) 



(ZOOL. CHALL. EXP. — PAKT XUII. — 1886.) Uu 2 



