MADEIRAN GROUP. 151 



sidering the number, and great extent, of the Porto-Santan 

 conchyliferous deposits, not a tenth part' of which have as yet 

 been thoroughly explored, there is absolutely no ground what- 

 ever for concluding that these few examples, which have as yet 

 been brought to light, occupy a position in any degree different 

 from those of the other species with which they are associated, 

 or that they require to be accounted for by methods of trans- 

 mission, during the remote past, concerning which we can form 

 no kind of idea that rises above the merest speculation. " Mais 

 d'ou est venue,' says Mr. Watson, ' et quand est venue cette 

 coquille ? Une coquille morte, abandonnee par un oiseau, 

 meme a une epoque prehistorique, ne suffit pas pour faire placer 

 1'espece au nombre des formes indigenes.' For my own part I 

 cannot but think that no apology is required for the occurrence 

 of these three examples of the H. lapicida in the subfossiliferous 

 beds of Porto Santo ; and indeed I shall be much suprised if 

 some future explorer in the island does not exhume the species 

 in far greater abundance. 



I may just mention that the Porto-Santan examples of the 

 H. lapicida have been examined with the greatest possible care 

 by Mr. Lowe, Mr. Watson, and myself, with all the desire (if it 

 were possible) to detect some peculiarity about them sufficient 

 to justify their separation as a distinct species, and that they 

 correspond in every particular with the more northern type. 1 



( Callina, Lowe.) 



Helix rotula. 



Helix rotula, Lowe, Cambr. Phil. S. Trans, iv. 53. t. 6. 



f. 10 (1801) 



Pfei/., Mon. Hel. i. 216 (1848) 

 Lowe, Proc. Zool. Soc. Lond. 183 (1854) 

 Alb., Mai. Mad. 28. t. 6. f. 16-18 (1854) 

 Paiva, Mon. Moll. Mad. 82 (1867) 



Habitat Portum Sanctum ; in montibus vulgaris. In arena 

 calcarea Helicifera hinc inde semifossilis parce reperitur. 



The H. rotula (which measures about 6 lines across its 

 broadest part) is one of the commonest of the Helices of Porto 

 Santo, to which island it is peculiar. It may be known by its 

 solid substance, its depresso-conoidal, acutely-keeled form, its 

 small and nearly closed-up perforation, its rather numerous and 

 flattened volutions, and by its entire surface being sculptured 



1 Mr. Lowe, in reference to this point, says : Din et sedulo scrutanti, ad 

 amussim cum exemplaribus Britannicis recentibus exemplar vel optime con- 

 servation fossile hoc pretiosissimum, mihi comparand! causa benignissime 

 commissum, omnino congruere compertum est.' (Proc. Zool. Soc. Lond., 1854, 

 p. 107). 



