441 



LIMN^EA. 



LIMN^AD^E. 



442 



a prismatic structure, aa in Yorkshire. The colours of limestone vary 

 indefinitely. When argillaceous matter is mixed with the calcareous 

 basis of the rock, the colour generally approaches to blue ; magnesian 

 and oolitic limestones are often yellow ; primary limestone and chalk 

 are generally white ; the Tiree marble is red ; some of the Derbyshire 

 and Kilkenny marble is black ; and there are many veined and parti- 

 coloured marbles, as those of Babbacombe, Sienna, Ac. 



Limestones contain a very large proportion of the organic bodies 

 which diversify the stratified rocks, few except the early primary 

 limestones being wholly deficient of shells, corals, fishes, &c. Occa- 

 sionally shells and zoophytes contribute to the beauty of particular 

 marbles, as the shell marble of Carinthia, Purbeck, &c., the criuoidal 

 marble of Derbyshire, and the coralliferous limestone of Weardale. 

 [MOUNTAIN LIMESTONE FORMATION.] 



LIMN^EA. [LIMS^AD^:.] 



LIMN-iEAD^i, a family of fresh-water Testaceous Mollusca, con- 

 sisting of the genera Planorbis, Limrwea, Physa, and A ncylta. 



Two of these forms (Planorbis and Limiuea) were included by 

 Linnaeus under his great genus Helix ; the third was arranged by him ! 

 among the heterogeneous assemblage of testaceous animals, which he 

 placed under his genus Sulla. Ancylus is placed in this family by i 

 Forbes and Hanley. Miiller separated the first of these under the 

 name of Planorbis, and the second under the name of Succinum, a 

 name already pre-occupied by Linnxus for a genus of Marine Testa- 

 ceous Gastropods entirely different, and Lamarck changed the name 

 to Lymncea, or, as it should be more correctly written, Limncea. 

 Adanson appears to have been the first who established the genus 

 afterwards named Phyia by Draparnaud, and the former gave it the 

 appellation of Bulin. Bruguieres followed Mttller as far as regards 

 Planorbis, but he placed the other two forms under bis genus Bulimns. 

 [BULIMCS.] 



Lamarck collected these three genera in the following order, Planar- 

 bit, Phya, and lAmntta under one family, his Lymne'ens, with the 

 following definition : 



Amphibian Trachelipoda, generally deprived of an operculum, and 

 having flattened tentacles. They live in fresh-water, and come to 

 respire the air at the surface. 



Their shell is spirivalve, most frequently smooth on the external | 

 surface, and always having the right edge of its aperture sharp, and 

 not reflected. 



The general opinion seems to be that these three genera are well | 

 associated in forming the family Limnaadce. 



( 'uvier, though he gives them no common family name, places the 

 three genera together, observing that the Planorbes are the faithful 

 companions of the Limmea in all our stagnant waters. 



M. De Blainville makes his first family of Pulmolranchiata(Limnacea) 

 consist of these three genera ; and M. Rang, retaining Lamarck's name, 

 places the Limne'ens, consisting of the same genera, as the fourth 

 family of the Pulmone's Inopercul5s of FeVussac (Pulmobranches of 

 De Blainville). 



Mr. O. B. Sowerby however is of opinion that the genera Physa and 

 Limnaia ought not to be separated. He observes ( Genera. Limnea,' 

 No. 8), that he finds himself obliged either to unite two genera which ! 

 have appeared distinct to Lamarck and Draparnaud, and which have 

 been adopted by some succeeding writers, or, contrary to his wishes, 

 and, as he thinks, to the interests of conchological science, we must 

 not only separate the Phyite from the Limnceae, but we must also adopt 

 Dr. Fleming's Aplexa, and Dr. Leach's Myxat, each of which would, j 

 aa far as we yet know, only contain one species. These, he adds, are 

 all fresh-water shells; and the only describable difference in the shells, 

 except mere specific differences, consists in the Aplexa and Physa, 

 being heterostrophe shells, while the Limncea, and Myxas are dextral. 

 Greater differences he acknowledges are found in the animals, chiefly 

 in their tentacula and in their mantles ; the Myxas of Leach and the 

 Physa of Draparnaud having the power of extending the edges of 

 their mantle over a large portion of the external part of their shell, 

 which the Limncca of Lamarck and the Aplexa of Fleming have not, 

 while the tentacula of all but Physa are compressed and triangular, 

 and even in Physa they are compressed according to Lamarck, though 

 filiform : in all of them the eyes are found at the internal base of the 

 teutacula, supported on very short tubercular pedicles. He concludes 

 by uniting the whole of these genera under the generic appellation of 

 Limnwa, and divides them into four sections, thus : 



1. Shell very thin, subglobose, polished ; internal lip dilated ; aper- 

 ture ovate, dextral. Animal with the mantle reflected ; the tentacles 

 short and trigonal. Myjas, Leach's manuscript ; Helix ylutinosa, 

 Mont. ; Limnrea glutinoia, Drap. 



2. Shell thin, obovate, polished ; internal lip dilated, the aperture 

 ovate or ovato-lanceolate, sinistral. Animal with the mantle reflected; 

 the tentacles subulate. Physa, Lamb. ; BMa, Linn. 



3. Shell thin, oblong, polished ; the internal lip equalling the external, 

 the aperture lanceolate and sinistral. Animal with the mantle not 

 reflected ; the tentacles trigonal. Aplexa, Fleming ; Physa, Drap. ; 

 Bulla Injpnorum, Linn. 



4. Shell thin, generally oblong, rather solid; the aperture oval and 

 dentral, the inner lip equalling the external one. Animal with the 

 mantle not reflected, the tentacles compressed and trigonal. Lymncea, 

 Lam.; J/elis, Linn. 



M. Deshayes rejects this opinion, and retains Physa as a genus, for 

 reasons which the reader will find under that head in this article. 



Returning to Lamarck, we find him remarking upon the cause 

 which led to the peculiar organisation of his Lymne'ens, in accordance 

 with one. of his favourite fanciful theories. It would seem, says he, 

 that those fluviatile Trachelipods, which inhabited waters of little 

 depth, such as those of small rivers, ponds, and marshes, which are 

 exposed to the accident of being dried up, were often reduced to live 

 in mud more or less desiccated. They then found themselves forced 

 to habituate themselves to the air, to breathe it. This habit having 

 modified their branchiae, like those of the Colitnaee's, is become to 

 them a matter of necessity ; so that though living in the water, they 

 are now obliged to come from time to time to its surface in order to 

 breathe the free air. This circumstance in their manner of life seems 

 to have had its influence in rendering an operculum useless to them ; 

 and they are in general deprived of one. Those fluviatile Trachelipoda, 

 on the contrary, which we know to be unable to respire anything but 

 water, have all an operculum. . 



It is only necessary to reflect for a moment on the principle involved 

 in these suppositions, to reduce them to their true value. 



Leaving his theoretical views for his practical observations, we find 

 Lamarck thus neatly pointing out a leading character for distin- 

 guishing the family. "The Limneans have only two tentacles; they 

 are flattened, and never oculated at their summit." 



M. De Blainville thus defines his family Limnacca : 



Body very variable in form ; two tentacles eminently contractile, 

 carrying sessile eyes at the internal side of their base. 



Shell delicate, with the external border constantly trenchant. 



He further observes that the animals of this family are always found 

 in fresh waters, stagnant or running, often at their surface, and some- 

 times in their depths. The shell he remarks, presents very variable 

 forms. He arranges the genera in the following order : Limncea, 

 Physa, Planorbis. 



M. Rang gives a more extensive definition of the Limnaceans of 

 Lamarck (Limnacea of De Blainv. ; Limnocochlides, without a collar, 

 of Latreille), thus : 



Animal elongated, having the body distinct from the foot, and 

 twisted spirally backwards; never any buckler (or cuirass), but a 

 collar formed all round the neck by the edge of the mantle ; head 

 surmounted by a sort of veil which is very large ; tentacles two in 

 number, the eyes differently situated at their base ; pulmonary cavity 

 showing its orifice upon the collar ; organs of generation separated ; 

 anus near the orifice of the lung. 



Shell always complete, very much rolled up (tres enroulde), 

 delicate, and with the external border or lip trenchant. 



All fluviatile. 



M. Rang arranges the genera in the following order : -Plunorbis, 

 Limncea, Physa. This is the order given by Cuvier, and, as far as 

 these three genera are concerned, by Draparnaud, who however makes 

 Ancyha intervene between Planorbit and Limmeia. 



Planorbis. Animal elongated, compressed, slender, and very 

 strongly rolled up ; head furnished with two tentacles, which are 

 contractile, setaceous, very long, and oculated at their internal base ; 

 mouth furnished superiorly with a crescent-shaped tooth, and belo~.v 

 with a lingual mass armed with small hooks, and surmounted by a 

 sort of veil which is short and notched ; foot oval and rather short ; 

 respiratory orifice on the left, upon the collar, and approximated by 

 that of the anus ; organs of generation separate, on the same side ; the 

 male organ near the tentacle, and the ovary at the base of the collar. 



Shell rather delicate, sinistral, very much rolled or coiled up on the 

 same p'ane ; concave on each side, the spire re-entrant (rentrant) ; 

 aperture rounded with a sharp border, and interrupted by the con- 

 vexity of the whorl which precedes it. (Rang.) 



Flanorliis, Shell and Animal, and eggs. 

 a, Planorbit carinatus ; b, mass of eggs of Planorlii cornem on a leaf. 



The species are widely diffused. Very few fresh waters, either 

 running or stagnant, are without some of the species. 



M. Rang remarks that the genus Planorbis offers a curious anomaly, 

 namely, that the animal as well as the shell is sinistral, and conse- 

 quently the orifices, instead of being situated on the right side, as in 

 other Gastropods, are placed on the left. 



Mr. Sowerby (Genera, No. 4) remarks that the principal peculiarity 

 in this genus appears to him to consist in the fact that the shells of 

 the genus are what are called reversed, a fact doubted by some, who 

 have described the species as umbilicated above. A careful examina- 

 tion of many of the species in a living state satisfied Mr. Soworby 



