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. Ferussac, 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 appears as 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 TESTACEULA. 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. 



