Mr. W. Clark on Ancylus oblongus and A. fluviatilis. 279 



standard of comparison with the A. fluviatilis and Limneus 

 pereger. 



Ancylus oblonguSj Brit. Moll. vol. iv. p. 188. pi. 122. fig. 5. 



Ancylus lacustrisy auct. 

 Velletia lacustrisj nonnuU. 



Shell an elongated, subdepressed, laterally compressed, coarsely 

 wrinkled light horn-coloured cone, with a posterior somewhat 

 horizontal vertex. 



Animal pale yellow drab, aspersed with dark lead- or cloud- 

 coloured minute points varying in intensity ; — we have thus at 

 the outset the characteristic colouring of the Limneadan tribe. 



Though the shell is of similar character with the A. fluviatilis, 

 the organs instead of being heterostrophe as in that species are 

 dextrorsal. 



The mantle is even with the margin of the shell and bounded 

 by a frosted line, within which are three irregular, somewhat 

 distant, peripheral cordons of minute, hyaline white, comma- 

 shaped fillets, the whole having the aspect of a spangled fringe : 

 this, and the form of the shell-cone, are the nearest approaches 

 to Patella, with which genus it has generally been associated by 

 the older authors. The head is flat and broad, springing from 

 a short thick neck, and forming a rather elongated hood or veil, 

 emarginate in the centre, so as to appear bilobed, having the 

 edges finely crenate : there are no head or neck lappets, but 

 there is a medial rib terminating in a small limited erect flap at 

 the anterior point of the head, beneath which is the crosial 

 puckered buccal orifice leading to a palate of two soft fleshy 

 lobes rounded in front and pointed behind ; between these a 

 tolerably long tongue, with the rachis garnished by extremely 

 close-set fine wiry laminse, that incline posteriorly like the 

 strands of a feather, and pass through the oesophagus into the 

 stomach. 



The two tentacula are contractile, short, flat, subtriangularly 

 tapering to a rounded termination, very divergent, with large 

 eyes in front at their bases, with rather an internal inclination. 

 The foot is a narrow elongated oval disk, rounded at both ends, 

 and has a very limited locomotion. 



The oesophageal collar or brain consists of two principal and 

 other smaller ganglia in close contiguity, which throw ofi" the 

 usual nervous threads, and on each sic^e of them are the pale 

 yellow subrotund lobular salivary glands. The muscular system 

 exhibits no peculiarity, distributing to every part of the body 

 the necessary strands, and displaying their interlacements at all 

 angles and planes. 



The stomach is white, submedially transversely contracted. 



