782 Nematura, a new Genus of Mollusca. [Dec. 



The shell is compressed laterally in a direction parallel to the 

 axis and to the plane of the aperture, as in scarabus, but the pro- 

 minent edges are rounded, and the former lips do not leave a ridge 

 or keel at each semi-turn as in that genus. The sudden deflexion of 

 the last whorl above the aperture, and the consequent depression and 

 constriction of the aperture is a singular feature in the shell, and, in 

 conjunction with its compressed form, led to my pronouncing the 

 species to be a type of a distinct genus before I became acquainted 

 with the animal or the operculum. 



The first specimens which I saw, were shewn to me by Mr. Cra- 

 croft, who found them destitute of inhabitant or operculum in the 

 Sunderbans east of the Jabuna river. He was unable to state whether 

 they were land or water shells. Their occurrence in the delta of 

 the Ganges so near to Calcutta, spurred me to diligence in the search 

 after the species, and, about a couple of months subsequently, I 

 discovered it alive between high and low water mark in pools, and 

 on wet mud recently left by the tide in the river Hooghly, immediate- 

 ly opposite to the Esplanade. Here they were accompanied by 

 Melania Virata* and Melania pyramis*. Dr. Pearson subsequently 

 found them in the mud deposited by the aqueduct which passes in 

 front of the Town Hall. Here we found them accompanied by assi- 

 minia fasciata, and by small specimens of novaculina gangetica. I 

 also took specimens attached to a floating bamboo in the Salt-water 

 Lake in company with Neritina depressa. 



Though found between high and low water mark like assiminia 

 fasciata, nematura does not otherwise resemble it in its habits, as it 

 never attempts to creep out of the water in which it is placed, but 

 ascending to the surface swims with the foot reversed in the same 

 manner as lymncea, planorbis, paludina, and the smaller melania use 

 to do. 



I have named the species on which the genus is founded 



Nematura\ Delta. Testa ovato-conica, a lateribus tumida, lute- 

 scente, ultimo anfractu ventricoso, majori omnibus oblique minute 

 striatis ; spira brevi ; apice acuto ; umbilico evanescente. Long. 

 0.25 poll. 



* Melania Virata, described as species D, and melania pyramis as species B, 

 in Gleanings in Science, vol. ii. p. 22. Species A of that paper, I have else- 

 where more fully described as M. variabilis, and I have named species C 

 M. elegans. 



f Lamarck has a genus of insects named Nemoura from vrj/xa filum and ovpa 

 canda ; but as he neglected the rules of composition the appellation of the 

 present genus, while it expresses a singular feature in the animal, will run no 

 risk of being confounded with that of the eminent naturalist. 



