SUB- GENUS HETEROTRYPA. 123 



recognised in longitudinal sections with difficulty ; but one 

 may occasionally detect the narrow cavity of one of these, per- 

 forating the thick fibrous wall between two of the ordinary 

 corallites (PL III. fig. if). 



M. tiunida, PhilL, is a common fossil In the Carboniferous 

 rocks of all parts of Britain, and the foregoing description is 

 founded upon specimens which I collected in the Carbonifer- 

 ous shales of Redesdale, Northumberland, where the species 

 is very abundant, and remarkably well preserved. Along 

 with the normal type of the species, however, I have met 

 with a limited number of examples which appear to me to 

 differ from the usual type so far as to be, at least, entitled 

 to separation as a distinct variety ; and I shall now proceed 

 to give a brief description of the characters of this : — 



Monticulipora tumida, Pliill., var. miliaria, Nich. 

 (PI. III. figs. 2, 2.-.) 



Corallum dendroid, about one-tenth of an inch in diameter, 

 branching at short intervals. Calices thick-walled, their aper- 

 tures varying from i-iooth to i-i20th inch in diameter, separ- 

 ated by interspaces occupied by the minute apertures of very 

 numerous interstitial tubuli, which are in places aggregated 

 into star-shaped maculse. These interstitial tubuli vary a good 

 deal in size, sometimes being mere circular dark spots (as seen 

 in section), sometimes exhibiting well-defined central cavities ; 

 but their number is always very great, and, even when not 

 aggregated into maculae, they commonly form a double row 

 between adjoining corallites. In other respects, this form 

 agrees entirely with the normal type of JM. tiunida, its coral- 

 lites being thin-walled in the centre of the branches, and 

 becoming thickened as they bend outwards to reach the 

 surface, their walls becoming at the same time completely 

 fused together, while their visceral chambers are crossed by 

 a moderate number of horizontal tabulse. 



In our ignorance of the complete corallum oi Monticulipora 



