] 00 PULMONIFERA. 



Genus PTEROSOMA, Lesson. 



Tentacles none ; eyes sessile on the head. Body gela- 

 tinous, transparent, sub-cylindrical, with a large, thin, hori- 

 zontal, wing-like lobe on each side. 



Shell none. 



Ex. P. plana, Lesson, pi. 70, fig. 5. 



M. Lesson discovered this curious genus, of which but 

 a single species is known, in the sea between the Moluccas 

 and New Guinea, where it was met with in great abundance; 

 he observes that it uses the membranous expansion with 

 which its body is surrounded for progression, and swims 

 horizontally with great velocity. 



Sub-class PULMONIFERA. 



Breathing organ in the form of an air-sac, or dorsal cavity, 

 lined by a vascular network. Heart situated before the lung ; 

 Abdomen rudimentary, or spiral and well-developed, usually 

 protected by a shell. Sexes separate, or united in the same 

 individual. Adult and larva shell -bearing, larva shaped like 

 the parent, not furnished with cephalic fins. 



Terrestrial or aquatic, respiring free air. 



The Vulmonifera differ from the families of Gill-bearing 

 Gasteropods in not undergoing a metamorphosis, the young 

 animal being shaped like the parent and not furnished with 

 deciduous cephalic fins. Their breathing organ, moreover, 

 is in the form of a sac, instead of being arranged in tufts or 

 folds, and they respire free air. In the Operculated division 

 the sexes are distinct, and the edge of the mantle is free 

 from the nape, but in those families in which the aperture 



