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 proper, 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), which are no less representative _inter se_. From which we are driven to conclude;--first, that this _quondam_ continent was densely stocked at the beginning with foci of radiation created expressly for itself[63]; and, secondly, that the areas which these various creatures had overspread, before the land of passage was broken up, was extremely limited,--or, which amounts to the same thing, that _their migratory progress was unusually slow_.