ZOOLOGY. 579 



The shells are mostly thin and homy, yet nevertheless 

 contain a considerable quantity of calcareous earth, and 

 are mostly devoid of opercula. 



Those species, which dwell in fresh water, do not 

 possess introvertible tentacula like the marine Snails, 

 and the eyes are placed at their basis ; the sexual orifices 

 are separate. 



In the Land-snails, the tentacula are introvertible, and 

 support the eyes upon their apex ; the sexual apertures 

 are blended into one. The former, like the marine 

 Snails, lay numerous small ova inclosed within a gela- 

 tinous mass in the water ; the latter deposit their ova free, 

 covered with a membranous, and occasionally calcareous 

 shell, in the earth. Copulation is effected in all by a 

 reciprocal interchange of the androgynous species. 



I have observed that individuals of Limnaea auricu- 

 laria can continue to propagate for several generations 

 without being impregnated ; they must impregnate them- 

 selves. 



Order 2. Conchozooid Snails Dicecii. 



3497. Branchiae situated within the cavity of the 

 mantle, and hanging down in the form of a comb ; shell 

 for the most part spirally contorted ; sexes separate 

 PectinibrancMata. 



Here belong the Capulidae, Turbinidae, Neritae, Conidae, 

 Volutidse, Buccinidae, Muricidae and Strombusidae. 



The teutacula are not retractile, and have the eyes 

 mostly seated at their basis ; the penis is external, very 

 large, and cannot be drawn within the body, but is only 

 reflected into the pallial cavity ; most of them have a 

 protrusile perforating proboscis, and an operculum. They 

 lay numerous small ova, contained within large mem- 

 branous cases, which frequently hang in rows to each 

 other like a .necklace of pearls. The shells are in some 

 instances horny, in others of a stony texture. 



Fain. 4. Mussel- Snails, Capulidce. 



Only one branchial comb within the mantle, and 

 covered merely by a flat hood-shaped shell ; no operculum. 



