154 THE GENUS MONTICULIPORA. 



actual surface (fig. 29, f), the walls of the large corallltes are 

 shown to be gradually diminishing in thickness as they pro- 

 ceed to the axis of the branch, while the interstitial corallites 

 become more clearly defined, and the dark spots which seem 

 to represent sections of spiniform corallites become much 

 reduced in number. 



In longitudinal sections (fig. 29, g), it is seen that the coral- 

 lites in the axis of the branch have quite thin and delicate walls, 

 the thickenincr not commencino; till the tubes beein to bend 

 outwards to the surface. Complete, horizontal tabulse are very 

 largely developed in the axis of the branches, and are even 

 more numerous in the circumferential parts of the tubes. 

 Lastly, I have found it difficult to recognise any distinction 

 between the larger and smaller corallites in longitudinal sec- 

 tions, and they do not appear to differ conspicuously in their 

 tabulation, or in any other point of internal structure. 



Though the mere external form and surface-characters of 

 M. Girvancnsis are not so strongly marked as to separate it 

 unmistakably from various other smooth types of the ramose 

 Moiiticiiliporcs, its internal structure is so peculiar that it is not 

 necessary to compare it closely with any other form with 

 which I am at present acquainted. 



This species is identical with the form which was noted by 

 Mr R. Etheridge, jun. and myself from the Craighead Lime- 

 stone (op. cit. supra) as an undetermined species of Chcetetes 

 (or Mouticuiipora). The material in our possession was ex- 

 ceedingly limited, and quite insufficient for specific determina- 

 tion ; and the additional specimens which have enabled me to 

 decide upon the distinctness of M. Girvancnsis, and to describe 

 it fully, unfortunately escaped notice until it was too late to in- 

 sert an account of the species in the Third Fasciculus of our 

 Monograph of the Silurian Fossils of Girvan. 



Horizon and Locality. — Lower Silurian (Craighead Lime- 

 stone), Craighead, near Girvan, Ayrshire. (Coll. Mrs Robert 

 Gray.) 



