{mperfectly-known Species of Stromatoporoids. 317 



" arras ^' of considerable thickness, thus enclosing subangular 

 zooidal meshes. 



Vertical sections (PI. X. fig. 2) show stout radial pillars, 

 united at about their own width apart by tolerably regular 

 " concentric laminae." About six pillars, with their inter- 

 vening spaces, occupy the space of 2 millim. measured trans- 

 versely. 



Obs. In the microscopic structure of the skeleton A. expari' 

 sum^ H. & W., closely approaches A. clathratum^ Nich., the 

 commonest species of Actinostroma in the European and 

 British Devonian rocks ; and it may be regarded as the 

 American representative of this species. It differs, indeed, 

 from typical examples of A. clathratum in no marked struc- 

 tural feature, except that the radial pillars are somewhat 

 stouter and are more irregularly developed. On the other 

 hand, A. expansum differs from A. clathratum in the appa- 

 rently constant feature that the skeletal layers are thrown into 

 slight but regular, circumscribed undulations, which give rise 

 on each successive lamina to low and broad eminences, the 

 centres of which are from 1 to 1^ centim. apart. The surface 

 thus acquires a characteristic aspect, which has been well 

 figured by Hall and Whitfield [loc. cit. suprh) and which is 

 not observable in A. clathratum. 



In the character just mentioned A. expansum presents a 

 general superficial resemblance to the fossil which I described 

 from the Corniferous Limestone of Ohio (Pal. of Ohio, vol. ii. 

 p. 249, fig. 3, 1875) as Stromatopora nodulata. Even super- 

 ficially, however, this resemblance is not complete ; and I 

 shall show later on in this paper that the latter form is proved 

 by its possession of a porous skeleton-fibre to belong in reality 

 to the genus Syringostroma, while its skeletal structure is much 

 coarser than that of A. expansum. 



Formation and Locality. Devonian (Chemung beds). Rock- 

 ford, Iowa. In the collection of the Geological Survey of 

 Canada are two specimens from the Devonian rocks of Lake 

 Winnipegosis (Island 52, Dawson Bay) which appear to 

 agree in essential respects with the present species. As the 

 internal structure, however, of these specimens is but imper- 

 fectly preserved, I have based the above description upon a 

 specimen from Rockford, Iowa, kindly furnished to me by 

 Mr. Whiteaves. 



Actinostroma Tyrrelliif Nich. 

 (PI. VIII. figs. 4 and 5.) 



The coenoBteum of this species (woodcut, fig. 1) is massive, 



