186 



SHELLS AND SHELL-FISH. 



are turbinated, and resemble those of Helios, although 

 the tip of the spire is acutely conical, and not, as in the 

 common snail, depressed. These obviously lead to the 

 typical sub-genus Cyclostoma; and these latter, again, to 

 the singular sub-genus Megalomastoma, another ad- 

 mirable group, detected and beautifully illustrated by 

 Guild ing ; it is the representative of Pupa, from which 

 it is distinguished by possessing a horny operculum. 

 Cyclotus of the same accurate zoologist is another 

 remarkable type : it is so much depressed, that it almost 

 resembles a Planorbis, and, from not having any pillar, 

 the umbilicus is open to the terminal whorl : the oper- 

 culum is shelly ; and although the aperture is round 

 and thickened, the inner lip is carried upwards in the 

 form of a little siphon : this is very remarkable in a 

 species we possess from India, and in another from the 

 "West Indies. We are disposed, indeed, to consider 

 this and Cyclophora to be the two typical forms of the 

 whole group which represent Planorbis , just as the 

 Lucernince do in the entire family of HelicidcB. 

 Guilding founded his group upon one species ( C. fus- 

 cescens}, which he found " with the spire corroded," in 

 the woods of St. Vincent ; and, unfortunately, all the 

 specimens found by him afterwards were dead shells, so 

 that the animal is still unknown. 

 Megalomastoma appears immedi- 

 ately to follow Cyclostoma, because 

 the M. suspensum Guild., in its 

 shell, is a Cyclostoma; but the 

 M. brunnea Guild., which is ob- 

 viously the type, is so elongated as 

 to resemble, at first sight, a Pupa. 

 The animal of M. suspensum is 

 often found suspended by glutinous 

 threads (fig. 29.); it has the mouth 

 rather elongated, proboscis-like, but 

 deeply cleft; the two tentacula 

 rather long, and the eyes at their 

 Having now concluded the survey of the four 



