152 MOLLUSCA+ 
and figured, which, however, differ remarkably from one 
another. 
Genus Limax.—Pulmonary orifice near the posterior 
margin of the shield. This genus, as now restricted by 
M. Férussac, differs from the Arion in the absence of the 
caudal mucous pore, the position of the pulmonary cavity, 
and by the orifice of the sexual organs being placed under 
the superior right tentaculum. The calcareous matter of 
the shield is more solid, and appearsas a shelly plate. 
In the following genus the shield is placed nearer to the 
tail than in the preceding group, and is fortified internally 
with a subspiral plate. 
Genus ParMACELLA.—Posterior extremity of the shield 
containing the shell. The pulmonary cavity is placed un- 
derneath the shell of the shield. This arrangement oc- 
casions a corresponding posterior position to the heart. 
Along the back, from the shield to the head, are three 
grooves, the middle one of which is double. The shield it- 
self adheres only at the posterior portion, the anterior part 
being free. The internal structure is similar to the slugs. 
The only marked difference, indeed, consists in two conical 
appendages of the sexual cavity, by which there is an ap- 
proach to the species of Helix. 
The Parmacella Olivieri is the best known species, and 
was first described, and its structure unfolded, by M. Cuvier. 
It was brought from Mesopotamia by M. Olivier. 
In the two following genera the cloak is destitute of a 
shield, and the pulmonary cavity is situated near the tail. 
Genus TresTACELLA.—Tail covered with a single spiral 
shell, underneath which is the pulmonary cavity. The vent 
and pulmonary cavity are, from the position of the protect- 
ing shell, on which they are dependent, nearly terminal. 
