36 UMN^IDAL. 



writers, as well as in the present day Gwyn Jeffreys, 

 have upon sufficiently strong grounds maintained that 

 it is undeniably dextral. 



To aid the reader in the identification of the species 

 I will again follow the method adopted by the author 

 of * British Conchology,' and divide them into 

 sections. 

 " A. Shell glossy, last whorl very large in proportion 



to the rest, and partly covering the preceding one." 



B.C., vol. i. p. 79. 



I. Planorbis linea'tus,* Walker. Pl. IV. 



Body very slightly transparent, brown with a reddish or violet 

 tinge above, of a paler colour underneath ; tentacles filiform, long, 

 slender, separated at their base, tolerably transparent, yellowish- 

 brown ; eyes small, not very prominent ; foot broad, rounded in 

 front, gradually tapering to a point behind. 



Shell quoit-shaped, compressed, but somewhat more convex 

 above than below, thin, exceedingly glossy, semitransparent, 

 yellowish or brownish horn-colour, sometimes grey, with close- 

 set transverse striae which are stronger and curved near the 

 aperture or mouth of the shell ; epidennis very thin ; periphe}y 

 obtusely keeled ; whorls 4, compressed, body whorl larger than 

 the rest of the shell, and overlapping a great part of the penulti- 

 mate whorl, provided internally with 2-5 rows of whitish, curved 

 ridges or plates, which are placed at nearly equal distances from, 

 and opposite to each other, so as to divide the interior of the 

 whorl into separate septa or chambers ; these plates are distinctly 

 visible through the shell ; sph'e deeply sunk ; suture shallow but 

 distinct ; mouth forming about two-thirds of a compressed oval ; 

 outer lip thin, flexuous, bluntly angular above ; umbilicus narrow 

 but deepish. 



Inhabits sluggish streams and ponds in the home 

 and eastern counties of England ; it occurs " in pro- 



* Having lines or ridges. 



