92 CONTRIBUTIONS TO CANADIAN PALEONTOLOGY. 



tubercle. Tabulae numerous, horizontal, at regular intervals apart of 

 •5 mm. Septa, sixteen in number, alternately long and short, the longer 

 passing half way to the centre and apparently not reaching the columella, 

 the smaller septa seen only in transverse sections and not recognizable in 

 the calyces. When the walls of the corallites are not in actual contact 

 the interstitial spaces are apparently filled with ccenenchymal tissue, the 

 exact structure of which has not been clearly ascertained although in 

 transverse sections it appears to be obscurely reticulated by faint linear 

 markings. 



Chazy limestone. — Mingan Islands, Que., W. E. Logan and J. Richard- 

 son, 1856. 



The similarity between the fossil from the Mingan Islands, described 

 by Billings, and Stylarcea occidentalism Nich. and Eth. jun., from the 

 lower Silurian rocks of the Girvan district in Ayrshire, Scotland, is 

 obvious to any one comparing the above description with the published 

 one of the latter species ; that they are specifically identical is probable, 

 but not having seen actual specimens of the Girvan fossil the writer is 

 unwilling to assert positively that they belong to the same species. 



Dr. Nicholson refers (op. cit. p. 95) the Craighead limestone near 

 Girvan and its associated shales from which S. occidentalis was obtained, 

 to a " tolerably low position in the Lower Silurian series, corresponding 

 perhaps with the upper part of the Trenton limestone or the base of the 

 Cincinnati and Hudson River formations of North America,"' and men- 

 tions being struck with " the strongly American fades of the Craighead 

 corals and in particular their resemblance to those of the Trenton and 

 Cincinnati groups." 



TETRADIID^]. 



Genus Tetradium, Dana. 1846. 



(Wilkes's Exped. Zoophytes, p. 701.) 



Corallum massive, composed of long, upright intimately united, thin 

 walled corallites that have a quadrangular or petaloid transverse section ; 

 septa springing from the centre of the walls, lamellar, stout at the base, 

 thin towards the edge, typically four in number, reaching about half way 

 to the centre of the visceral chamber ; secondary septa frequently present 

 near the angles, raising the total possible number to twelve ; tabuloe 

 numerous, complete, horizontal ; increase by fission of the corallites. 



