wHiTEAVES.] DEVONIAN FOSSILS, MACKENZIE RIVER BASIN. 211 



The size of specimens of the present species seems to be determined 

 by that of the objects to which they are attached. When growing 

 upon the surface of comparatively large corals the colonies attain to a 

 length of two inches or more, but when attached to the shells of 

 biachiopoda of medium size (such as Orthis striatula or Atrypa reticu- 

 laris), upon which they have no room to expand, the maximum length 

 of the colonies is usually less than an inch. 



The genus Hederella, which, so far as known, is exclusively confined 

 to rocks of Devonian age, was constituted by Professor James Hall in 

 1884 for the reception of a small group of cyclostomatous polyzoa of 

 the type of Aulopora Canadensis, Nicholson. Of the five nominal species 

 nowreferred to that genus, only two, viz., H. Canadensis and S. filiformis, 

 have as yet been found in Canada. 



In regard to the first of these it may be observed that, after a direct 

 comparison of authentic examples of both, the present writer is con- 

 vinced that IT. Canadensis is both generically and specifically identical 

 with the Stomatopora alternata of Hall and Whitfield, from the Devonian 

 rocks of Iowa. 



The second is a little creeping polyzoon which is abundant in the 

 shales of the Hamilton formation at Thedford and Bartlett's Mills, 

 near Arkona, Ont., and which was identified with the Aulopora filifor- 

 mis of Billings by Professor H. A. Nicholson in 1874. Subsequently, in 

 1881, Professor Hall, who seems to have taken the correctness of this 

 identification for granted, referred it to his then newly constituted 

 genus Hederella, under the name IT. filiformis. Unfortunately, how- 

 ever, the only desci-iption that Billings gave of his species is quite 

 insufficient to enable it to be recognized, and it was not accompanied 

 by any figure. The type and only known specimen of the Aulopora 

 filiformis of Billings, which is still preserved in the Museum of the 

 Survey and which is here figured for the first time (on Plate xxix., 

 fig. 1), appears to the writer to be the immature state of a species of 

 Syringopora allied to the S. fascicularis of Linnajus, from the Wenlock 

 limestone of Dudley and elsewhere. It is so completely silicifled that 

 all traces of its internal structure are obliterated, but, although its 

 corallites are at first creeping and form a large-meshed network, they 

 ultimately become erect, though only for a short distance, and are 

 more or less fasciculated. The Aulopora filiformis of Nicholson, 

 on the other hand, is unquestionably a Hederella, as pointed out 

 by Hall, and if sufficiently distinct from H. Canadensis, it will have 

 to be called H. filiformis, Nicholson. But, when a large number of 

 specimens of Hederellce are carefully compared, they are found to vary 

 so much in the amount of regularity of their mode of growth and 



