468 CHITOXIDiE. 



the median small ; the laterals large, with dentated hooks, 

 uncini five, trapezoidal, one of them erect and hooked. 

 Head surrounded by a semi-circular veil or hood ; eyes 

 and tentacles none ; mouth with cartilaginous jaws. 

 Gills, in a series of lamellae, between the mantle and foot 

 round the sides and posterior part of the body. Foot 

 oblong, rounded at each end. 



Shell composed of eight transverse valves or imbri- 

 cated plates, immersed in the coriaceous mantle which 

 forms an expanded margin beyond them. 



There are several anatomical peculiarities in these 

 animals, which have caused many eminent authorities to 

 hesitate even in considering them Mollusca. The number 

 of shelly valves covering their bodies gives them an arti- 

 culated appearance ; their heart is central and elongated, 

 like the dorsal vessel of the Annelida; their organs 

 of reproduction are symmetrical, and have two orifices ; 

 the sexes are united ; and the intestine is straight, with 

 the anal orifice posterior and median. These consi- 

 derations induced M. Blainville to constitute a special 

 group for them between the Mollusca and Annelida, and 

 which he named Polyplaxiphora. The opinion of Cuvier, 

 however, that they are Gasteropodous Mollusks is more 

 in accordance with nature ; their gills are like those of the 

 Patellidce ; their foot is that of a true Gasteropod ; and 

 in their lingual armature and stomach they resemble the 

 Prosohranchiate Mollusca; the extra portions of shelly 

 covering, moreover, are considered by Dr. Gray as so 

 many detached plates separated from the fore part of the 

 hind valve, which he regards as the homologue of the 

 shell of Patella. 



