﻿1861.] 



KIRKBY — -PERMIAN, SOUTH YORKSHIRE. 



319 



so slight an admixture of species is a matter of surprise, particu- 

 larly when an examination of the fauna of the Unter Zechstein, * 

 which appears to have been of contemporaneous deposition with the 

 Compact Limestone of Durham and the Lower Limestone of York- 

 shire, shows that 21 of the Yorkshire species (viz. 1 Cephalopod, 

 3 Gasteropods, 8 Conchifers, 1 Brachiopod, 3 Polyzoans, 4 Entomostra- 

 cans, and 1 llhizopod) were distributed in the German area during 

 the same period. 



In the Yorkshire fauna 9 of the species are Gasteropods ; in that 

 of the Compact Limestone there is only a single member of the same 

 class. In the Compact Limestone fauna 11 of the species are Bra- 

 chiopods; in the other there is only one. It is in these respects 

 that the two faunae differ most. Five out of the 9 Yorkshire Gonehifera 

 occur in the Compact Limestone : among them are the two common 

 species Ax. dubius and Gerv. antiqua ; but neither of these shells 

 is common there. The most common shell of the Compact Limestone 

 is Productus horridus. No species is so common as it ; Stroplialosia 

 lamellosa, Sjpirifera alata, and Gamarojohoria crumena being next in 

 the list of common species. Acanihocladia ariceps, the common Poly- 

 zoan of the Lower Limestone, is not rare in the Compact Limestone, 

 but the place of the common Polyzoan is there taken by Fenestella 

 retiformis. One of the Yorkshire Entomostraca is found in the 

 Durham subdivision. The Bhizopod Miliola pusilla is common to 

 both faunas, it being accompanied by a Dentalina in the Compact 

 Limestone. 



In these differences, in two contemporaneous assemblages of Per- 

 mian species, we have a good illustration of some of the peculiarities 

 that pertained to the distribution of marine life in palaeozoic times. 

 It is thus shown that, according to present researches, there is only 

 about one-third of the species of each fauna common to both groups, 

 thus leaving about two-thirds that are peculiar to each. And it 

 may be remarked that,, though all the species peculiar to the South 

 Yorkshire assemblage, except two, were afterwards distributed in 

 the Durham area during the Shell-limestone period, it woidd yet 

 appear that none of those peculiar to the Durham group were ever 

 common to the other. We can scarcely refer these differences 

 to the result of geographical distribution of species, for we cannot 

 but consider that the Permian deposits of Durham and Yorkshire 

 were accumulated in the same sea ; so that it would be perfectly 

 unwarrantable to ascribe differences so great in specific distribu- 

 tion to an agency of this kind, when the two regions were so 

 nearly situated and the nature of the sea-bottom the same. To 

 changes belonging to the distribution of species in depth, however, 

 the differences would easily seem referable; it being now well 

 known that great differences obtain in the distribution of marine 

 life as the zone of depth varies, even with short distances. I would 

 therefore refer the differences observed in these local faunae to a 

 change in the conditions of depth of sea existing in each area, while 

 their respective deposits were accumulating ; the difference being in 

 the greater depth of sea over the Durham area than to the south- 



VOL. XVII. — part i. z 



