180 HELIX. 



H. PLANORBELLA Lamarck, 1822. PI. 61, figs 5, 6. 



Umbilicate, orbicularly depressed, costulately striate, the costa3 

 arcuate, acute, smooth, thinner on the base, pale yellowish corneous, 

 trifasciate with gray, the first band subsutural, the second double, 

 just above, the third just below the periphery; spire slightly convex, 

 regular, apex flattened ; suture slightly impressed ; whorls 4i>, slightly 

 convex, subtumid near the suture, the last abruptly briefly de- 

 scending, obtusely angulate, constricted at the aperture, convex, 

 subinflated below, whitish in the umbilicus; aperture very oblique, 

 circularly-oval; peristome subacute, slightly reflexed, thickened 

 with white within, the margins much converging, joined by a thin 

 callus, the right margin arcuate, basal much thickened, appressed 

 in the middle outside, sub-straight within, colurnellar margin wide 

 and flatly callously reflexed. Diam. 20, alt. 10 mill. 



Ins. Teneriffe and Gomera, Canaries. 



H. drigata var. /?, Fer., and H. villiersii d'Orb. (pi. 64, figs. 6-8) 

 and of Reeve ? (pi. 61, fig. 12,) are synonyms. 



"According to Mousson, H. planorbella presents two totally dis- 

 tinct forms, one of them peculiar to Gomera, with the costre simple 

 or ungranulated, which he believes to correspond with the Lani- 

 arckian type, and the other, which is more depressed and keeled, 

 and has the ridges distinctly sculptured, to Teneriffe. This latter 

 phasis, figured by Pfeiffer as the -normal state of the H. planorbella, 

 Mousson treats as a Var. incisogranulata (pi. 61, fig. 7-10), and de- 

 fines it thus: 'umbilicus slightly narrower; plicae less numerous, 

 minutely granulate by transverse incised lines; basal margin less 

 adnate, less thickened, distinctly convex within.' " (Wollaston.) 



I have followed Mousson's identification of this species, and as I 

 have no specimens, have taken my description from his. Mabille, 

 however, whose work on Hemicycla is more exhaustive than any 

 other, and who is par excellence, analytical in his methods, considers 

 the "var. incisogranulata" the true planorbella. 



His note on this point, (under H. quadricincta, Nouv. Arch, da 

 'Mus. dHist Nat, 1885, p. 24,) is as follows: "Dans le planorbella, 

 lesstries costulees sont incisees exactement comme celles du plicaria; 

 le caractere que signale M. Mousson n'est reel qu'autant que Ton a 

 en vue une variete de 'cette espece a cotes lisses, variete qui nous 

 semble assez rare, et pourrait peut-etre, lorsqu'elle aura ete suffisam- 

 ment etudiee, constitutuer une espece distincte" I am unable to de- 

 cide upon the identity of a species upon which specialists on this 

 fauna hold such diverse opinions. 



