LAMARCK. 535 



Order V. HETEROPODES. No arms in coronet on the head ; 

 no foot under the belly or throat to creep upon. 

 One or several fins, not paired nor regular in their 

 position. 



Order I. PTEROPODES. 



There is no subdivision of this order which contains the 

 genera Hyalaea, Clio, Cleodora, Limacina, Cymbulia, and 

 Pneumodermon. 



Order II. GASTEROPODES. 



I. SECTION. Branchiae, whatever their position, raised 

 either in filaments, or laminae, or combs or tufts. They 

 breathe water only, (ffydrobranches.) 



(a) Branchiae external, placed above the mantle, either 

 on the back or on the sides, and never in a special cavity. 



Les Tritoniens. 



(b) Branchiae external, placed under the margin of the 

 mantle ; and disposed in a longitudinal series either 

 around the body or on one side only, but not in a special 

 cavity. 



Les Phyllidiens. 



Les Semi-Phyllidiens. 



(c) Branchiae placed in a special cavity on the back, 

 situated forwards near the neck. 



Les Calyptraciens. 



(d) Branchiae placed in a special cavity towards the pos- 

 terior part of the back, and covered either by the 

 mantle or by an opercular shield. 



f No tentacula. Les Bulleens, 



-ff With tentacula. Les Laplysiens. 



II. SECTION. Branchiae creeping, under the form of a vas- 

 cular network, on the walls of a special cavity, the aperture 

 of which the animal opens and shuts at pleasure. They 

 breathe the atmosphere only. (Pneumobranches.) 



Les Limaciens. 



Order III. TRACHELIPODES. 



I. SECTION. T. without a prominent siphon, and in general 

 breathing by a hole. The most part of them are herbivo- 

 rous and furnished with jaws. Shell with the aperture 

 entire, having at its base neither a dorsally directed emargi- 

 nation nor canal. (Les Phytiphages.) 



* Trachelipods breathing the air. Shell spiral, even, 

 not distinctly nacred. 



