38 limn^idje:. 



The shell of this species is thinner and of a paler 

 colour than that of P. lineatus, the spire is more raised, 

 the keel is sharper, and the interior of the body whorl 

 is not divided into septa or chambers. 



B. Whorls feiv, 

 3. P. NAUTI'LEUS,* LiNNfi. PL. IV. 



Body greyish-brown, rather paler underneath, dotted with 

 small black specks ; /^^^^ very thick ; tentacles nearly cylindrical, 

 rather thick, transparent, light grey, considerably separated at 

 their base ; eyes moderately large, round, black ; foot greyish- 

 brown with a reddish tinge, rounded in front, narrow but 

 terminating obtusely behind. 



Shell discoid, depressed, flat or slightly concave above, rather 

 convex beneath, thin, subpellucid, of a dullish aspect, brownish, 

 or greyish, occasionally white ; epidermis thickish ; periphery 

 slightly and obtusely keeled ; whorls 3, body whorl laiger than 

 the rest of the shell, furnished at regular intervals with strong 

 curvilinear ridges, which are often armed with spinous crests ; 

 suture deepish ; mouth oval, or somewhat circulai-, oblique ; 

 outer lip thin, forming a complete peristome with the inner one ; 

 umbilicus large. 



Inhabits lakes and ponds throughout Great Britain, 

 attaching itself to aquatic plants. This is an ex- 

 ceedingly beautiful little creature ; the shell, as its 

 name implies, bears a strong resemblance to some of 

 the Nautili, which inhabit tropical seas. It is usually 

 to be found on the under side of the leaves of water 

 plants, particularly those of the water lily when they 

 are partially decayed. It is an inactive species ; when 

 crawling, the animal carries its shell inclined to one 



Resembling a nautilus. 



