£06 Dr. WJieelton Hind — Correlation of Carboniferous Rochs. 



iiuinerons in the Cambrian than at later periods, and all the Cambrian 

 blind genera belong to this order. In this formation the blind, 

 genera constitute also a much larger proportion of" the whole trilobitic 

 fauna than they do in higher PalEeozoic beds. These facts indicate 

 that the abundance of blind genera at this geological period marks 

 a certain stage in the evolution of the Trilobites, and not necessarily 

 the pi-evaleuce of certain conditions which induced as an adaptive 

 modification the loss of eyes. In the Ordovician we notice that 

 all the blind genera which do not belong to this Hypoparian type 

 are the more or less morphologically primitive ancestors of the higher 

 families, and still possess many features associated with the lower and 

 earlier Cambrian forms. Larval characters are retained in the adult 

 in spite of secondary specialization, and are accompanied by signs of 

 progressive differentiation ; as, for instance, in Areia and Placoparia. 

 These primitive and ancestral genera have died out when we reach 

 the Silurian, with the exception of one belated survivor. We no 

 longer find the Hypoparia present in any force, and therefore these 

 trilobites, belonging to the phylogenetic stage anterior to the 

 evolution of compound eyes, are absent. All the blind forms which 

 do occur belong to the higher families and genera, which normally 

 possess well-developed eyes. Eyes have been lost in a few species 

 lor special reasons for which we have now to seek ; and we cannot 

 explain their absence on phylogenetic grounds. 



Character of the Roclcs. 

 The character of the rock in which blind genera and species 

 belonging to both our groups have been found has received con- 

 siderable attention at the hands of some palteontologists, and much 

 importance has been attached to it. But since we have shown that 

 all the genera in Group 1 owe their want of eyes to their phylo- 

 genetic position, and that the absence of these organs in their case 

 cannot be interpreted as a secondary adaptive character, no question 

 about the manner and materials of deposition apparently concerns 

 them. This has not been always recognized, and indeed it is 

 possible to argue that the persistence and survival of these blind 

 genera of primitive organization were due to the existence of con- 

 ditions under which eyes would have been of little or no use. For 

 this reason we must look more closely into the character of the 

 sediment which accumulated, the physical conditions which pre- 

 vailed, and the fauna which flourished during the existence of the 

 blind genera of our Group 1. 



{To be continued.) 



IV. — On Mk. W. Gcnn's Correlation of the Carboniferous 



EocKs OF England and Scotland. 



ByWHEELTON Hind, M.D., B.S. Lond., F.R.C.S., F.G.S. 



ALL students of Carboniferous geology will hail with pleasure 

 Mr. W. Gunn's masterly paper in the August number of the 

 Geological Magazine (p. 342), and it is only to be regretted that 

 the publication of such important stratigraphical facts should have 



