1892.] 



THE BRITISH NATURALIST. 



155 



fresh-water species the effect of too great heat produces a frail, delicate 

 and dwarfed shell, and extreme cold produces the same effect. L. 

 peregra var. thermalis which lives in water said to reach 95^' F., and 

 the var. glacialis living in water but little removed from the freezing 

 point even in summer, both exhibit these characteristics. The Rev. 

 W. C. Hey states that the warm water discharged into the river Foss 

 caused the Anodons at that part to be more delicate and larger than 

 elsewhere, '-exemplifying the genial effect of" (moderate) "heat upon 

 forms of life." Among the slugs, melanism is stated to be effected by 

 extreme cold. Herr Simroth has shown that this character is 

 developed in A. atev by cold, and redness by warmth, as seen in A. 

 rufits, a common variety on the continent. L. arhoruni, says Mr, 

 Taylor, "gives similar testimony in the north-western parts of this 

 country ... on the Italian mountains the transition from the 

 ordinary to the intensely dark form can be traced." — W. A. Gain. 



The Genus Rissoa — (continued) — Sub-genus Manzonia, Brus. 

 Jeffreys in his report on the "Lightning" and "Porcupine" 

 expeditions proposes a genus Flemingia for the two species— 7?. 

 costata and R. zetlandica — wliich we are about to consider. Either 

 this becomes a synonym of the earlier Manzonia described by Brusina 

 in the "Journal de Conchyliologie " for 1868, or may be retained for 

 the latter species by the strictest systematists who prefer to group this 

 shell separately. The two species however seem to fall naturally 

 together, as in Jeffrey's group B, by the thickening or doubling of 

 their outer lip (called the peristome), which forms a complete and 

 slightly raised rim round the mouth. Both species give one the 

 impression of being twisted aside and form a very distinct little group. 

 ITryon assigns it eight species — all European; this is probably far too 

 large a number, as besides our two representatives only two others 

 seem to have been assigned to I Manzonia, both from | Madeira. 

 Rissoa zetlandica, Mtg. is a scarce shell, mostly found in dredgings 

 though I have taken it dead on the shell-beach at Herm amongst the 

 my xidids oi R. cancellata. It may be distinguished from this and all 

 other species by being turreted — almost scalariform — in shape, with a 

 produced spire and a light clear yellow colour if fairly fresh. The 

 shell is strongly sculptured with ribs across and lengthwise at right 

 angles, forming square pits, while the ribs towards the point of the 

 shell gradually dwindle down to mere striae. The distinguishing- 

 features to look for are the turreted whorls with their strong concella- 

 tion and deep suture, the double or reflected outer lip, and the oblique 

 twist given to the last whorl and aperture. Anyone collecting in the 

 Channel Isles should look out for it at Herm but it is certainly a deep- 



* P.Z.S., 1884, p. 116. t Struct, and Syst., Conch. II., 263. % i'-^ S.. 1873, p. 371. 



