PULMONATA. 83 



The Limnsese are inhabitants of freshwater streams and pools, and occasionally of 

 brackish marshes. The living species are found in all parts of the world, but 

 principally in the temperate zones. In the fossil state, species have been found in the 

 Wealden formations ; and they occur in great profusion in the freshwater deposits of 

 the Eocene epoch, and, in greater or less abundance, in nearly all the lacustrine 

 formations above those deposits. In England, as in the Paris basin, the fossil Limnseae 

 occur in very different conditions in the various deposits ; in the limestone of the 

 lower formation, called, from the abundance of their remains, the " Limnsean Lime- 

 stone," specimens with the shell preserved are very rare ; generally only the casts are 

 found, the shelly matter having been absorbed. In the upper marls they occur in 

 great profusion, and. although very fragile, usually in a beautiful state of pre- 

 servation. 



It is exceedingly difficult, as both Lamarck and De Blainville have observed, to 

 distinguish the different species ; the length of the spire, the contour of the volutions, 

 and the size and shape of the aperture, characters by which species may be separated 

 with tolerable certainty, in other genera, are, in this genus, exceedingly variable, and 

 glide by imperceptible gradations from one extreme to another ; so that reliance 

 cannot be implicitly placed on them. The character which appears to exhibit the 

 least variation is the columellar fold, although this also occasionally presents con- 

 siderable differences in form and condition. By this character, however, the genus 

 may be divided into two groups, one comprising the species in which the fold is 

 flattened ; the other consisting of the species in which it is rounded or sub-acute. 

 Each of these groups may, again, be subdivided into two sections, according as the 

 upper parts of the whorls, forming the sides of the spire, are convex or flat. By the 

 use of these artificial distinctions, the separation of the species will be much 

 facilitated. 



Sect. a. Columellar fold compressed, generally bipartite. 



No. 30. Limn^a caudata. F. E. Edwards. Tab. XII, fig. 2 a — c. 



L. testa ovato-acutd, ventricosd, lavi : anfractibus numerosis, convexiusculis, ultimo 

 penultimoque rapidc cresceniibus : ultimo obsolete et irregulariter corrugato : spird conico- 



' squeezed up' between two very large lateral ones, each primary lateral having a very large apex internally, 

 with a small external one, while, at the edge, they have altered to one thick prolonged apex projecting 

 inwards, and irregularly lobed on its upper edge. Much the same arrangement prevails in Amphipeplea, 

 where, however, the tubercle of the lateral teeth is even still larger in proportion to its plate. Physa, again, 

 exhibits a multitude of teeth of a similar form, though different to any that I have seen in other genera." 

 The dentition of Aplexus is not described. 



