30 7^. G. Carmfhern — A Revmon of some Carboniferous Corah. 



is more pronounced, tlie cardinal fossida only being denoted by the 

 shortness of the cardinal septum. 



In transverse sections the distinctive characters of the variety are 

 well expressed. Here the curvature of the major septa is seen to be 

 extremely even and regular throughout. The counter septum and its 

 neighbours are prominent, being generally longer (PI. IV, Figs. 7, 

 la), though sometimes shorter (PL IV, Pig. 8) than the rest. 

 Nevertheless, the interseptal chambers on each side of the counter 

 septum rarely enlarge to form a counter fossula, though one, or both, 

 may be so elongated as to reach the centre of the dense mass of fused 

 sei)ta lying in the centre of the corallum ; this dense central area is 

 of somewhat greater size than that observed in Z. omaliusi. 



The inconspicuous cardinal fossula has a characteristic shape, 

 narrowing inwardly, instead of slightly expanding as it does in the 

 normal species. This occurs even in the young stages of growth, 

 though sometimes masked by the insertion of young major septa. 



Vertical sections show that the tahulee are essentially of the same 

 simple type as those in the normal species, but there is only one 

 depression on their surfaces, corresponding to the single fossula 

 usually present in this variety. 



Xocalittes. 



Upper Visean : Crosshouse (Lower Limestone Group of Scotland) 

 (D) ; Thornton and other quarries near Colne, Lancashire 

 (D ?) ; Warsaw Knoll, near Clitheroe (D ?) ; middle and upper 

 part of horizon Y, Burrington, Mendips. 



Tournaisian : Big Weston Wood Quarry, Portishead, near Bristol 

 (Zg subzone), very rare ; coast at Malahide, co. Dublin 

 (loo. I), and Eush Slates, E. 4 «, E 6 «, and E 8 «. 



Hemarks. 



The points of difference between this variety and Z. omaliusi 

 proper are certainly not great. Nevertheless, these differences are so 

 constant and are so readily detected in a hand- specimen that in my 

 opinion they clearly merit varietal distinction. The variety forms an 

 intermediate link between Z. omaliusi and Densiphyllids of the type 

 of D. charlestonensis, Thorn.' When the septal curvature is but 

 slightly developed it becomes a niost difficult and often, so far as I 

 can see, an impossible matter to separate the two corals. Such cases 

 must, however, be expected in dealing with considerable assemblages 

 of similar forms. 



It is tempting to suppose that Z. omaliusi, Z. omaliusi var densa, 

 and D. charlestonensis represent so many stages in one line of 

 evolution, more especially when it is remembered that in the Bristol 

 district var. densa is chiefly found just above the maximum of 

 Z. omaliusi, and B. charlestonensis occurs much higher up in D0-D3 

 of the neighbouring district of the Gower and Oystermouth, and is 

 common on a probably similar horizon in Scotland and is again found 

 in the shales overlying the Derbyshire Limestone. Nevertheless, 



1 J. Thomson, "Corals Carb. Syst. Scot.": Proc. Phil. Soc. Glasgow, p. 152, 

 pi. vi, figs. 21, 22, etc. (1883). 



