170 LITTORINIDiE. 



longitudinal line, rounded behind, without a caudal fila- 

 ment, upper lobe of foot very little, perhaps not at all, 

 extended laterally beyond the pedal disk. Near its ex- 

 tremity it bears a yellow, horn-coloured, subcircular oper- 

 culum, the nucleus is a slight depression, the plate is not 

 spiral, being composed of very fine elliptic strife of in- 

 crement. The animal is not at all shy, shows its organ 

 freely, marches with quickness and vivacity, carrying its 

 shell sometimes at an elevation of near 80", and often 

 swimming with the foot uj^permost.'' 



Mr. Clark remarks that this animal can hardly be placed 

 in the genus Rissoa. It seems to us to belong to the sub- 

 group Hydrohia, and to be an extreme form of marine 

 Bissoee, analogous to B. anatina, among the brackish 

 water species. 



Under the name of Planaria, two shells, said to have been found on the 

 coast near Dunbar, the first of which reminds us by its figure (pi. 8, f. 53, 54, 

 55) of S. nitidissima, the other (pi. 8, f. 48, 49) of Planorbis albtis, are thus de- 

 scribed (p. 21) in Brown's " Illustrations." 



" P. pellucida. Shell depressed, very thin, pellucid, white, and extremely 

 glossy, consisting of rounded volutions, slightly wrinkled across, visible on both 

 sides of the disk ; the one next the bodj' above nearly parallel with it, the inner 

 ones descending into a deep umbilicus ; the centre volutions encompassed by the 

 body ; aperture semilunar, transverse, and oblique ; outer lip thin, pillar-lip 

 adhering to, and slightly reflected on, the columella ; base rounded. Diameter 

 a quarter of an inch ; thickness not an eighth." 



" P. alba. Shell depressed, with four milk-white rounded volutions visible on 

 both sides, encompassed by the body; the second ones very slightly elevated 

 above the sides of the body, the central ones sinking into a deep umbilicus ; 

 base rounded, the volutions retiring towards the middle, and winding to a small 

 central umbilicus ; the whole crossed by very minute lines of growth or obsolete 

 strife ; aperture oblique, large, suborbicular, and white within ; outer lip thin 

 and plain ; pillar-lip adhering to the columella. Diameter three-eighths of an 

 inch, and about half that thickness." 



