﻿1861.] 



KIRKBT PERMIAN, SOUTH YORKSHIRE. 



305 



6. Axinus dubius, Schlotheim, Denksch. Akad. Miinch., vol. vi. 

 p. 31, pi. 6. figs. 4, 5. PI. VII. figs. 11 and 12. 



Syns. Ax. obscurus, Sowerby; Ax. parvus, pusillus, productus, 

 undatus, elongatus, rotundatus, and Lucina minima, Brown ; 

 Schizodus Schlotheimi, Geinitz ; and Sch. truncatus, King. 



The most common form of this species in Yorkshire is that which 

 has been described by Sowerby under the specific name of "obscurus." 

 Its distinguishing features being already so well known, it will be un- 

 necessary to notice them in detail. It differs from other varietal 

 forms of this species in its greater depth anteriorly and in its greater 

 posterior production, in the convexity of its umbonal region and 

 larger umbone, in the obliquity of its hinge-line, and (generally) 

 in the thickness of its shell. 



Specimens from some localities — especially those from Moorhouse 

 — show several deep farrows and ridges in the concavity of the um- 

 bonal region, running somewhat radiately from the umbone, which 

 are probably visceral markings. Such examples also generally show 

 traces of the muscular impressions. These internal markings are, 

 of course, best seen in specimens with thick shells, and are very 

 faint or not at all perceptible when the shell has not been of mode- 

 rate thickness, as at Emsall and Brodsworth. The thickness of the 

 shell has been quoted as a specific character by those who consider 

 "obscurus " a distinct species ; but it may be pointed out that, though 

 the shell of the Yorkshire Axini is usually much thicker than the 

 shell of those occurring in Durham, yet its thickness is subject to 

 variation, just as the size of the shell is variable. Some individuals, and 

 those not young ones, have the shell little or no thicker than that of 

 the " truncatus " variety of the Shell-limestone of Durham, whereas 

 in other cases the shell is fully three times as thick. Thus, instead 

 of the difference in shell-thickness being of specific value, it would 

 seem to be merely a difference of individual growth, due probably 

 to age and locality, which are known to effect analogous results with 

 the shells of recent Mollusca. 



It would appear that Ax. dubius attained its maximum develop- 

 ment in the British area in the sea of the Lower Limestone. It was 

 there of greater size, more robust, and more prolific than at any 

 other period of its history. In no other district, and on no other 

 horizon, in Yorkshire, Durham, or any other part of Britain where 

 Permian strata occur, does it ever again attain the same size and 

 abundance. During the same period — while the Compact Limestone 

 was accumulating — it was a rare species in the Durham area. And 

 this is a fact worthy of attention : for the two regions are only some 

 eighty miles distant ; yet in one it is the commonest species, and in 

 the other amongst the rarest. 



The Axini of the Brotherton Beds also belong to the " obscurus " 

 type ; but they are there much dwarfed in size, seldom exceeding 

 half-an-inch in width. 



In some localities another variety of this species occurs (figs. 11, 

 12), which is much smaller than the preceding, its width being only 

 •j^jths of an inch. It is almost ovate in marginal outline, is regu- 



t2 



