Potamopyrgus.] GASTROPODA. 235 



Diameter, 1-25 mm. ; height, 2-75 mm. 



Animal unknown. 



Type in my collection. 



Hob. The only specimen was obtained by Mr. W. W. Smith, of 

 Ashburton, in 1892, by pumping water from a well 48 ft. deep. 



Remarks. The shell is exceedingly fragile, and distinguished, like 

 P. pupoidcs, by its subcylindrical form. The nearest allies are egenus 

 and spelceus, but it differs considerably from both. It most likely has 

 been derived from the latter species, and its subterranean habitat is 

 no doubt a long-established one. 



Fam. THIARIDJE. 



Mdaniidce, Gray. 



Animal having the rostrum large, well developed, sinuate in front. 

 Tentacles subulate, with the eyes on bulgings at their outer sides. 

 Foot large and short, subtruncate, with a rudimentary siphonal fold 

 in front, obtuse posteriorly. Mantle-margin generally fringed. In- 

 tromittant organ behind the right tentacle ; reproduction viviparous. 

 A single gill, composed of rigid, cylindrical plates. Central tooth of 

 radula trapezoidal, margin multicuspidate ; lateral tooth narrow, with 

 a broad anterior plate, and a number of cusps, the median one being 

 larger ; marginals long and narrow, multicuspid. 



Shell spiral, generally fruireted, covered with a thick dark-coloured 

 epidermis ; aperture often channelled or emarginate below ; outer 

 lip simple, sharp ; operculum horny, ovate, spiral or sublamellate ; 

 nucleus subcentral or marginal. 



These animals are fluviatile, being inhabitants of fresh-water lakes 

 and rivers throughout the warmer parts of the world. 



Genus 1. MELANOPSIS, Ferussac pere, 1807. 



Melanopsis. Ferussac, " Essai d'une methods conchyliologique," 1807, 70. 

 Type : M. costata, Fer. Faunus, Megerle, not of Montfort. Carnpeloma, 

 Rafin. Ceneona, Gist. Bulliopsis, Conrad. 



Animal having the tentacles long and pointed. Mantle-margin not 

 fringed. 



Shell ovate, last whorl elongated, smooth or longitudinally plicate ; 

 spire short, acute ; aperture oblong, distinctly notched in front ; 

 inner lip thick, with a posterior callus ; outer lip simple, acute ; oper- 

 culum paucispiral or subspiral ; nucleus submarginal, terminal. 



Distribution. New Zealand, New Caledonia, Spain, northern 

 Africa, Asia Minor. 



This genus dates back to the Cretaceous ; it is widely distributed 

 in the Tertiary of Europe, and several species are known from the 

 Miocene of the United States (Maryland). 



