318 



Prof. H. A. Nicholson on some new or 



conical, or hemispherical in form, sometimes of large size. 

 Though the coenosteum splits with great facility into thicker 

 or thinner concentric strata, the mode of growth is not defi- 

 nitely latilaminar. The surface is covered with minute and 

 close-set tubercles representing the upper ends of the radial 

 pillars, and also exhibits numerous small, few-branched astro- 

 rhizse, the centres of which are from 3 to 5 millim. apart. 

 The astrorhizse are superimposed in vertical systems and the 

 axial canals of these commonly open on minute, slightly 



Kg. 1. 



*?%v 



Side view of an exfoliated specimen of Actinostroma Tyrrettii, Nich., of 

 the natural size. The specimen is one in which " mamelons " are 

 but incompletely developed, and the basal portion has been cut off. 

 Devonian, Lake Winnipegosis, Canada. (Coll. Geol. Survey of 

 Canada.) 



elevated, conical prominences or " mamelons," the summits of 

 which are also 3-5 millim. apart. In some specimens, how- 

 ever, these " mamelons " are very faintly marked, or, indeed, 

 may not be recognizable at all. 



