IIYIUTINA. / 



columella straight, solid, twisted, and obliquely truncate 



in front ; outer lip acute, thickened internally. 



St/11. Aplustra, Strains. 



Ex. A. aplustre, Linnaeus, pi. 50, fig. 3. Shell, A. 

 aplustre, fig. 3, a. 



The animal of this genus is very similar, both in 

 markings and colour, to that of Hydatina, but the shell, 

 in place of being convolute, with the axis pervious as far 

 as the apex of the spire, has a straight and solid colu- 

 mella, and the fore part of the aperture is slightly chan- 

 nelled. The only species known is from the Mauritius, 

 where it seems to prefer the muddy banks of the mouths 

 of the rivers. 



Genus HYDATINA, Schumacher. 



Shell involute, thin, ventricose, axis pervious as far as 

 the spire, smooth, banded, covered with a thin, horny 

 epidermis ; spire depressly flattened, last whorl inflated ; 

 aperture expanded and entire in front ; inner lip mem- 

 branaceous, adnate to the body-whorl ; columella ar- 

 cuated, reflexed, continued anteriorly into the outer lip ; 

 outer lip thin, simple, acute. 



Ex. H. circulata, Martyn, pi. 50, fig. 4. Shell, II. 

 physis, Linnteus, fig. 4, a. 



The animal of this genus is adorned with beautiful 

 colours, and when viewed, fully expanded and crawling 

 among the bright green alga3 in clear water over a sandy 

 floor, exhibits a beautiful appearance. Unlike that of 

 Aplustrum, the shell in this genus is loosely convolute, 

 the inner lip forming a spiral canal similar to that in 

 ikera. The whorls are handsomely banded and marked 



