PULMONIFERA. 101 



of the shell is not closed by an operculum, both sexes are 

 united in the same individual, and the edge of the mantle is 

 united to the nape. 



The lingual membrane in the Pulmonifera is very short 

 and broad, and there is great uniformity in the general ar- 

 rangement of the teeth. The teeth are very numerous and 

 similar, and are placed in numerous transverse rows, which 

 are straight in the Helicida and Amphibolidie, diverging 

 from the centre in Testacellidce, slightly curved in Sip ho - 

 nariida, angularly bent near the centre in Auriculida, and 

 semilunar in Cyclophoridm. In the strictly-herbivorous 

 tribes the teeth are similar in form, with broad bases and 

 dentate crowns, as in Limneidce and Hclicidw, but in the 

 carnivorous families the teeth, on the contrary, are more or 

 less pointed, as in Olsacinida and Testacellidce. 



The shell in the air-breathing Gasteropods, according to 

 the researches of Gegenbaur, is not secreted on the exterior 

 of the mantle as in other Gasteropods, but is deposited, in 

 the form of calcareous granules, within its substance, a mode 

 of development, as Huxley observes, which is without parallel 

 among other Gasteropodous families, and, with the excep- 

 tion of the Cephalopoda, is unknown among the Mollusca. 

 In some genera the shell is entirely wanting, as in Onchi- 

 dium; in others it remains rudimentary and granular, as 

 in Avion ; in some again it is internal, flat, and partially 

 membranous, as in Limax ; in others it is external, small, 

 and ear-like, as in Testacella ; or it is partly internal, and 

 partly external and shield-like, as in Parmacella ; while 

 in the greater number it forms a more or less developed 

 spiral cone, into which the animal is enabled to retract. 

 The cone may be elevated and with many coils, as in the 

 higher genera of a series; or reduced to a simple, depressed 



VOL. II. I- 



