BELLINI : FRESHWATER SHELLS OF NAPLES AND NEIGHBOURHOOD. 39 



Var. gibilmannica O. G. Costa. — "Corrisp. Zoologica," p. 113, 

 1839; Phil, "Moll. Sic," II., p. 121, t. 21, f. 5 (Z. solidus). A 

 cylindrical shell; five whorls slightly channelled, the lowest angular; 

 deep sutures. 



Z. peregra is a very variable form and we may number as many 

 varieties as habitats.^ This and the preceding species are modifica- 

 tions of same specific type, whose innumerable variations have been 

 distinguished by more than 300 names. 



The genus Limncea Brug., 1791, comprises forms very variable and 

 united by intermediate links. The simplicity of aperture, the uni- 

 formity of color, the variability of columella, even in species of the 

 same genus and locality, the difficulty of observing the animals are 

 reasons that hinder the deduction of sure conclusions. The habitat 

 also is of little importance, because a species may reproduce in every 

 region of the earth ; Z. mtnnta, Z. stagnalis, L. miricidaria and L. 

 peregra gives us a very good example. Consequently specific characters 

 are established on the proportions of the spire and on the extension 

 of the last whorl. 



The Limncece live upon aquatic plants and ascend on the branches 

 for respiration on the surface of the water; often they turn over and 

 by their locomotor train of mucus go traiUng along the surface of the 

 water as on a solid body. When the water is low and they cannot 

 submerge themselves they adhere by the mouth to a submerged body 

 and close it by mucus. Some species live also out of the water, like 

 Z. peregra; others may frequently crawl for days on plants and moist 

 ground. 



In LimncecB, as in other freshwater shells, we often note the erosion 

 of the shell ; this phenomenon may be hereditary, and Locard^ thinks 

 it an anomaly of individuals that pass a certain time out of water and 

 have the shell incrusted by earthy substances and cryptogams; 

 Gassies"^ supposes the cause of this corrosion to be an aquatic Myria- 

 pod, and Fischer^ believes that the molluscks take the calcareous 

 matter from other shells, when they need it. He has made convincing 

 experiments on this subject. 



(8) Planorbis fontanus Lightfoot— Z7z//. Trans., XXVI., I., 

 p. 165, t. ii., f. I, 1786; Drap., "Hist.," p. 47, t. ii., f. 20-22 {non 

 Stud.). In limpid waters on plants (Villa Nazionale, Naples). 



1 Taylor, J. W. : "The variations of Limncea peregra Miill.," /. Conch., vol. 7, p. 2S4, 

 October, iSgo. 



2 "Etudes sur les variations malacologiques," etc., vol. 2, 1881, p. 479. 



3 "Tableau m^thodique et descriptif des mollusques de I'Ag^nais," p. 167. 



4 Joxirncil de Conchyliologie., 1852, vol. 3, p. 303. 



