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fact that two of them (for the third has apparently 

 become extinct) have not altered one iota since the fossil 

 period, which,, in the opinion of Sir Charles Lyell, is 

 anterior to the dissolution of the intermediate land ; 

 whereas,, had they been mere modifications of each other, 

 induced by the local conditions and influences to which 

 they have been, through a long series of ages, severally 

 exposed,, the difference between their recent contour and 

 that of their fossil homologues would have been doubtless 

 at once conspicuous. I gather, therefore, that like the 

 Tarphii, to which we have lately drawn attention, they 

 are veritable surviving members of an esoteric assemblage 

 which found its birth-place on this post-miocene (?) tract. 

 In a similar manner, the H. undata in Madeira pro- 

 per, the H. Vulcania on the Dezertas, and the H. Porto - 

 sanctana in Porto Santo, are representative species,- 

 each occupying the same position, and being equally 

 abundant, on their respective islands : and, although it 

 may be a problem whether the second of these is not an 

 insular modification of the first (or vice versa) ; yet, with 

 the analogy of the three already mentioned before us, I 

 am inclined a priori to view it as distinct. These, also, 

 occur in a subfossil state ; and no alteration appears to 

 have been brought about, by either circumstances or 

 time. And so it is with numerous others (as the H. latens 

 in Madeira, and the H. obtecta in Porto Santo; the 

 H. squalida in Madeira, and the H. depauperata in 

 Porto Santo; the H. Delphinula in Madeira, and the 

 H. tectiformis in Porto Santo), widen are no less repre- 



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