MOLLUSCA I GASTEROPODA. 265 



and Atlantida, the former characterised by having a small shell 

 covering the circulatory and respiratory organs, or by having 

 no shell at all ; whilst in the latter, there is a well developed 

 shell, into which the animal can retire, and an operculum is 

 often present. 



SUB-CLASS B. PULMONIFERA OR PULMOGASTEROPODA. In this 

 sub-class of the Gasteropoda respiration is aerial, and is carried 

 on by an inflection of the mantle, forming a pulmonary cham- 

 ber, into which air is admitted by an external aperture. The 

 flexure of the intestine is neural, and the sexes are united in 

 the same individual. 



The Pulmonifera include the ordinary land- snails, slugs, 

 pond-snails, &c., and are usually provided with a well deve- 

 loped shell, though this may be rudimentary (as in the slugs), 

 or even wanting. Though formed to breathe air directly, 

 many of the members of this sub-class are capable of inhabiting 

 fresh water. They are divided into two sections as follows : 



Section I. Inopezmlata. Animaljnrt^rgyided with an oper- 

 culum to close the shell In. this_section are included the fami- 

 lies Helicidce ifLand-snails). Limacidce (Slugs), Oncidiadce, 

 Limnceidce (Pond- snails), and Auriculidce. 



Section II. Operculata. Shell closed by an operculum. In 

 this section are included the families Cyclostomidce, and Acicu- 

 lidce. 



SYNOPSIS OF THE FAMILIES OF THE GASTEROPODA. (AFTER WOODWARD.) 



SECTION A. BRANCHIFERA. Kespiration aquatic, by the walls of the mantle- 

 cavity, or by branchiae. 

 ORDER I. PROSOBRANCHIATA. The branchiae situated (proson) in advance 



of the heart. 



Division a. Siphonostomata. Margin of the shell-aperture notched or 

 produced into a canal. 



Fam. 1. Strombidce. Shell with an expanded lip, deeply notched 

 near the canal. Operculum claw-shaped. Foot narrow, adapted 

 for leaping. 111. Gen. Strombus, Pteroceras. 



Fam. 2. Muricida. Shell with a straight anterior canal, the aperture 

 entire posteriorly. Foot broad. 111. Gren. Murex, Triton, Pyrula, 

 Fusus. 



Fam. 3. Buccinidce. Shell notched anteriorly, or with the canal 

 abruptly reflected, producing a kind of varix on the front of the 

 shell. 111. Gen. Buccinum, Nassa, Purpura, Cassis, Harpa, Oliva. 



Fam. 4. Conidte. Shell inversely conical, with a long, narrow aper- 

 ture, the outer lip notched at or near the suture. Operculum 

 minute, lamellar. 111. Gen. Conus, Pleurotoma. 



Fam. 5. Volutida. Shell turreted or convolute, the aperture notched 

 in front ; the columella obliquely plaited. No operculum. Foot 

 very large ; mantle often reflected over the shell. 111. Gen. Voluta, 

 Mitra, Marginella. 



Fam. 6. Cyprceidce. Shell convolute, enamelled ; spire concealed, 

 aperture narrow, channelled at each end. Outer lip thin in the 



