812 CLASS XIII. 



in form of a long leaf. TREVIRANUS, accordingly, compares this animal 

 with Pleurobranchus, but remarks at the same time that in this last genus 

 the gill lies on the right side, and has many transverse folds, whilst that of 

 Ancylus presents an entirely smooth surface. (Zeitschr. fur Physiologic, iv. 

 i, 1822, s. 192 197, Taf. xvii.). TROSCHEL refers Ancylus to the Hypo- 

 branchiata (see WIEGMANN'S Archiv, 1838, n. s. 285). But since, however, 

 VOGT has subsequently remarked, that these animals appear to breathe air 

 alone, and that it has by no means been proved that the leaf-like organ is 

 really to be regarded as a gill (MUELLER'S Archiv, 1841, s. 25 28), we still 

 think that Ancylus must remain amongst the pneunionica aquatilia. 



Limnceus Cuv., Lymncea LAM. Tentacles broad, short, com- 

 pressed, triangular, not retractile. Eyes placed between the bases 

 of tentacles. Shell thin, oblong, sometimes turrite; spire exsert. 

 Aperture entire, longitudinal. Labrum acute, ascending towards 

 columella and forming an oblique fold. 



Mud-snail. These animals have a thin shell; the last wreath is very 

 wide and round. On the right side of the animal is the respiratory aper- 

 ture, the walls of which can be elongated as a tube, and thus extend above 

 the water. Such is the case when the animal respires at the surface of the 

 water, with the foot or the ventral disc turned upwards. The apertures of 

 the sexual organs lie far apart. Under the right feeler is the aperture 

 which transmits the penis, under the respiratory aperture that of the female 

 organs. To this it is to be ascribed that in copulating one individual is 

 connected with two others, one of which impregnates it whilst the other is 

 impregnated by it. In this way, hanging together, they often form several 

 long chains. (Vox BAER has, however, observed self-impregnation also in 

 Limnceus auricularis, MUELLER'S Archiv, n. 1835, s. 224.) These snails 

 live on water-plants. 



Compare SWAMMERDAM Bijbel der Nat. I. bl. 164 169, Tab. ix. fig. 4; 

 CUVIER Ann. du Mus. vn. pp. 185 193, PI. x. figs. 2 TI, Mollusq., Mem. 

 No. 14; STIEBEL Dissert, inaug. sistens Limnei stagnalis Anatomen, Got- 

 tingse, 1815, 4to, c. tabulis 2. 



Sp. Limnceus stagnalis, Helix stagnalis L. (703), BLAINV. Malac. PI. 37, 

 fig. i ; PFEIFFER Schn. I. Tab. iv. fig. 19, &c. 



Sub-genus AmpliipepUu NILSSON. Mantle involving shell. Shell 

 subglobose ; spire obtuse, scarcely prominent. 

 Sp. Limnceus glutinosus DRAPARN. 



Physa DRAPARN. (Bulinus ADANS.) Tentacles slender, seta- 

 ceous, long. Foot long, anteriorly rounded, posteriorly narrow, acute. 

 Margins of mantle sometimes fimbriate, reflected over shell. Shell 

 oval or oblong, often left-handed ; aperture longitudinal, produced 

 anteriorly, enlarged. 



