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. fiir Physiologie, Iv. 
2, 1822, s. 192—197, Taf. xvi1.). TROSCHEL refers Ancylus to the Hypo- 
branchiata (see WIEGMANN’S Archiv, 1838, 11. s.285). But since, however, 
Vocr 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 prewmonica aquatilia. 
Limneus Cuy., Lymnea 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. (Von Bakr has, however, observed self-impregnation also in 
Linneus wuricularis, MuELuER’s Archiv, 1. 1835, s. 224.) These snails 
live on water-plants. 
Compare SWAMMERDAM Bijbel der Nat. 1. bl. 164—169, Tab. rx. fig. 4 ;— 
CuviER Ann. du Mus. vil. pp. 185—193, Pl. X. figs. 2—11, Mollusq., Mém. 
No. 14;—SrIEBEL Dissert. inaug. sistens Limnei stagnalis Anatomen, Got- 
tingee, 1815, 4to, c. tabulis 2. 
Sp. Limneus stagnalis, Helix stagnalis L. (703), Buainv. Malac. Pl. 37, 
fig. 1; PFEIFFER Schn. 1. Tab. Iv. fig. 19, &e. 
Sub-genus Amphipeplea Niusson. Mantle involving shell. Shell 
subglobose ; spire obtuse, scarcely prominent. 
Sp. Limneus 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. 
