170 TEST ACE A ATLANTIC A. 



( Discula, Lowe.) 



Helix tetrica. 



Helix tetrica, Paiva, in litt. 



Loive, Ann. Nat. Hist. x. 95 (1862) 



Pfei/., Mai. Bldtt. xi. 53 (1864) 



Paiva, Mon. Moll. Mad. 87. t. 1. f. 7 (1867) 



Habitat Desertam Australem ; in prseruptis excelsis mariti- 

 mis rarissima, inter lichenes latitans. Semifossilis parce repe- 

 ritur. 



This is one of the largest and most distinct members of the 

 Discula section (measuring about 7^ lines across its broadest 

 part), and one which seems to be quite unconnected, so far at 

 least as our present data would imply, with any of the numerous 

 varieties of the protean H. polymorpha. It was detected on 

 the Southern Deserta (or Bugio), ' amongst lichens on the sea- 

 cliffs, in the spring of 1861,' by a man who was sent out by the 

 Baron Paiva to collect for him on that remote rock ; and it ap- 

 pears to have been extremely scarce even there. It had however 

 been previously obtained in a subfossil condition by Mr. Lowe, 

 who met with a single example of it during our visit to that 

 island in June of 1855. 



Apart from its larger size, solid substance, and flattened, dis- 

 coidal, lozenge-shaped form, the H. tetrica may be recognised 

 by its extremely wide, open, spiral umbilicus, its not very 

 strongly pronounced keel (which however is placed rather above 

 the dorsal line), and by the coarse and greatly elevated granules 

 with which it is everywhere uniformly asperated. In colour too 

 it is most peculiar, the fasciae (in at any rate the few examples 

 which I have had an opportunity of inspecting) being so broadly 

 developed as to cause nearly the entire surface, except the paler 

 and yellowish umbilical area, to be of a dark reddish coffee- 

 brown (which makes the white tubercles remarkably conspicu- 

 ous). Its aperture, which is of a dingy reddish-brown within, 

 has the peristome (though not perfectly circular) a good deal 

 elevated, the upper and lower portions of it being joined across 

 the body-volution by a thick corneous process. 



Helix polymorpha. 



Helix polymorpha, Lowe, Cambr. Phil. S. Trans, iv. 54 



(1831) 



Pfei/., Mon. Hel. i. 213 (1848) 



Alb., Mai. Mad. 25 (1854) 



Paiva, Mon. Moll. Mad. 83 (1867) 



Habitat ins. omnes Maderenses ; sub lapidibus, praecipue in 



