IANTHLMLU:. 85 



their testaceous coverings; but in Pterotrackeida, in which 

 the body is very large, the shells are rudimentary, or even 

 entirely absent. M. Blainville considered the Heteropods 

 (which he styled Nucleobranchiata) as a single family of 

 the Nudibranchiate Order; M. Cuvier regarded thern as 

 constituting a particular Order of Gasteropods; and Milne 

 Edwards believes that they form an abnormal group, or 

 second section of Swimming Gasteropods, of the same 

 value as the Ordinary Gasteropods, which are characterised 

 by a large, fleshy, flattened foot, formed for crawling on the 

 earth. Unwilling, however, to separate the Branchiferous 

 tribes, which undergo a metamorphosis and live in the 

 water, from the Pulmoniferous Mollusks, whose young are 

 like the parent and which respire free air, we have preferred 

 to view the Heteropods as an independent Sub- Class be- 

 tween these two great divisions. 



Fam. IANTHINID^E. 



Tongue, with the rachis unarmed ; the lateral teeth nume- 

 rous, uniform, simple, slender. Head proboscidiform ; ten- 

 tacles short and obtuse, with pointed eye-pedicels at their 

 bases, but without any trace of eyes. Gills plumose, par- 

 tially exserted. Foot small, flat, rudimentary, furnished 

 with a vesicular appendage on the hinder part. ? Sexes 

 separate. 



Shell thin, translucent, spiral, more or less turbinate, with 

 a sinistral nucleus. 



Pelagian. 



The armature of the lingual membrane in these oceanic 

 animals somewhat resembles that of Scalidce, which also 

 have a similar power of secreting a purple fluid; the lingual 



VOL. II. N 



