MADEIRAN GROUP. 169 



tains of Porto Santo, particularly the Pico Branco, where it 

 absolutely swarms, towards the summit, within the crevices of 

 the rocks and upon the stems of shrubby plants, especially a 

 native wall-flower (the Cheiranthus arbuscula, Lowe). In a 

 subfossil condition it appears to be scarce, though I have met 

 with it sparingly on the Campo de Baixo and also on the Ilheo 

 de Baixo ; but I am not certain that it has been found on the 

 latter adjacent islet in a living state. 



Apart from its elevated column and subconical contour, the 

 present Helix may be known by the tumidity of its volutions, 

 which are so prominent as to form a kind of obtuse keel (imme- 

 diately above the suture) which is usually traceable up the spire, 

 by its umbilicus, although not large, being open and deep, by 

 its surface being coarsely granulated both above and below (the 

 granules, however, being often sub-evanescent about the most 

 prominent part of the whorls), and by its peristome being con- 

 tinuous though not circular, the upper and lower lips being 

 joined at their insertion by a thick corneous callosity. The 

 shell is extremely solid in substance, or incrassated, and nor- 

 mally of a faintly plumbeous white with two narrow darker 

 bands beneath (one of which is sometimes absent), and another 

 above, just under (and adjoining) the suture, and continued 

 for a considerable distance up towards the apex. These three 

 bands are occasionally broken-up, or even well-nigh obsolete ; 

 but it is scarcely necessary to establish ' varieties ' and ' subva- 

 rieties ' (so-called) upon trifling fluctuations of mere colour. 



There is, however, a distinct phasis of the shell, which was 

 detected by myself towards the northern coast of Porto Santo, in 

 the district known as ' Pedragal,' and towards the Pico Juliana, 

 which deserves notice, inasmuch as it is so permanently dif- 

 ferent from the typical one as to have been described by Mr. 

 Lowe as a separate species, under the name of H. mustelina. I 

 think there can be no question that the intermediate races 

 which occur connect it with the true cheiranthicola type ; never- 

 theless it is a little smaller than the latter and more uniformly 

 and roughly granulated all over, its volutions are flatter (or 

 much less tumid) and with hardly any indications of a keel, its 

 umbilicus is joined (though not exactly overlapped) by the more 

 angularly produced lamina of the lower lip, and its colour is 

 more dingy, the surface being less evidently fa sciated (though 

 sometimes with an obsolete medial band above), but crowded 

 with irregular brownish fragmentary markings. 



