MADEIRAN GROUP. 61 



however (manifest though it be), is immeasurably overbalanced 

 by a consideration of how radically the respective faunas do in 

 reality differ from each other in the vast majority of their ab- 

 solute details. Thus, to take the Madeiras, which more parti- 

 cularly concern us in this section, the most distinctive forms are 

 peculiar to the Group, not only as regards the species but even 

 as regards the very types. Look, for example, at the section 

 Coronaria, of the genus Helix, and Hystricella, both of 

 which stand isolated, and apart, as pre-eminently Madeiran ; or 

 the little assemblages to which Mr. Lowe applied the names of 

 Placentula, Actinella, Rimula, and Caseolus. Or, to instance 

 the larger modifications, there is the Tectula department, as 

 well as Helicomela, Katostoma, and Cryptaxis, all of which 

 are restricted to the archipelago. And, apart from the true 

 Helices, facts are not wanting which would likewise tend to sepa- 

 rate, as it were, the Madeiras, at any rate to a considerable ex- 

 tent, from the other islands. Thus, the genus Clausilia is not 

 only well expressed there but literally universal ; yet it is with- 

 out so much as a member at the Azores, Canaries, and Cape 

 Verdes ; and the true Cyclostomas, which are so greatly deve- 

 loped at the Canaries, have in the Madeiras no single represen- 

 tative. This latter circumstance however is quite in harmony 

 with the Helicideous section Hemicycla, which is altogether 

 unknown at Madeira, but which numbers 37 exponents (indeed 

 probably more) in the neighbouring Canarian Group. 



As I have already mentioned in the prefatory remarks to this 

 volume, there are certain spots, and even small districts, scat- 

 tered here and there throughout these several Atlantic archipe- 

 lagos, which may be defined as essentially subfossiliferous ones. 

 They are either calcareous (partaking sometimes of the nature 

 of sand-dunes), under which circumstances the specimens are for 

 the most part more completely subfossilized, or else muddy, 

 as though composed of earth and refuse which had been washed, 

 at some remote period, into their present positions, by the 

 action of sudden and violent floods ; in which latter case the 

 shells, although generally more brittle and broken up, are less 

 altered, being totally unthickened, and presenting at times 

 faint traces of even colour. The nature, and probable age, of 

 these sedimentary beds I do not propose to discuss, for they 

 scarcely enter into my exact subject, and moreover they involve 

 considerations of great geological difficulty such as even Sir 

 Charles Lyell was not able satisfactorily to grapple with when he 

 examined those of Madeira and Porto Santo, now many years 

 ago, with considerable care ; but since it is pretty evident, I 

 think, that some of them must have been deposited previous to 



