MALACOZOA. GASTEROPODA. PULMOBRANCHIATA. 117 



perceptible longitudinal striulse ; the outer whorl rounded, but 

 flattened on the periphery so as to form a rounded angle on 

 the upper and an obtuse angle on the lower margin of the disk; 

 mouth nearly square, but with the upper angle more rounded. 

 Diameter two twelfths and a half, height half a twelfth. 



This variety, although closely allied to Planorbis Vortex, 

 is easily distinguished from it. Compared with young indi- 

 viduals of that species of the same size, it is found to be some- 

 times twice the height or thickness, it being, although not 

 much more than half the diameter, thicker than even a full- 

 grown vortex. Its glossy, delicately striolate surface is very 

 different from that of the other shell ; its periphery is almost 

 flat, although slightly sloped, in place of being rounded and 

 considerably sloped ; and its mouth is either square, in young 

 individuals, or square with the angles rounded, in old ones. 

 The turns are more convex above, as well as much more so 

 beneath, where the cavity is much deeper. At first sight ono 

 might almost mistake it for a large Planorbis contortus. The 

 colour of the shell is a rich yellowish-brown, like that of 

 Zonites nitidulus. 



The animal is similar to Planorbis Vortex, but with the lobes 

 of the veil less deeply separated, and the tentacula less dilated 

 at the base. Its habits are similar. 



It occurs in the same ditch as the last species, where I first 

 found it at the same time. 



The next species or variety is connected with Planorbis 

 Vortex by the present, which, if merely a variety, belongs as 

 much to the one as to the other. 



B. Planorbis Vortex Spirorbis. Bound-edged Coil-Shell. 



Shell extremely depressed, very thin, pellucid, glossy, regu- 

 larly and equally concave above and beneath ; with five, some- 

 times six, gradually increasing volutions, which are convex 

 above and beneath, and transversely obsoletely striolate ; the 

 outer whorl rounded on the periphery, forming an obtuse angle 

 on the lower margin of the disk ; mouth nearly square, a little 

 oblique, sometimes roundish. Diameter nearly three-twelfths 

 of an inch, height a third of a twelfth. 



It varies considerably in the form of the aperture, the degree 

 of carination of the margin, and the thickness ; but may always 

 be distinguished from PL Vortex by having the turns fewer, 

 smaller, more finely striate, and more glossy ; and from Plan- 

 -M-bis Crassulus, by being larger and much thinner. At the 



