imperfectly -knoicn Species of Stromatoporoids. 319 



The internal structure is that of a typical Acttnostroma. 

 Vertical sections (PI. VIII. fig. 5) show that the coenosteum 

 is composed of close-set and comparatively slender radial 

 pillars, which are united at short intervals by very closely 

 arranged " concentric larainai," formed by the section of the 

 horizontal connecting-processes. About twelve radial pillars 

 and about eighteen or twenty " concentric laminae " occupy a 

 space of 2 millim., measured respectively transversely and 

 vertically. 



Tangential sections (PI. VIII. fig. 4) show a well-marked 

 " hexactinellid " structure, the rounded and close-set ends of 

 the transversely divided radial pillars being connected by 

 well- developed whorls of connecting-processes or " arras," 

 which enclose minute, angular, zooidal meshes. 



Obs. In its general form and superficial aspect, as well as 

 in many of the details of its internal structure, A. Tyr- 

 rellii presents a very close resemblance to A. steUulatum, 

 Nich. It may therefore be regarded as representing in the 

 American continent the latter very common and characteristic 

 species of the Devonian rocks of Europe and Britain. In 

 fact there are no external or obvious features by which these 

 two forms may be distinguished, except that in A. Tyrrellii 

 the astrorhizai are decidedly more closely set than in A. steUu- 

 latum (in which they are often 6-8 millim. apart), and that 

 the centres of these structures are commonly placed on small 

 and markedly pointed " mamelons," such eminences in A. 

 SteUulatum being, when present at all, low and flat. This 

 distinction does not, however, amount to much, seeing that in 

 both forms " mamelons " may not be developed at all. 



On the other hand, A. Tyrrellii differs markedly from A. 

 SteUulatum in the following details of internal structure : — 



a. The horizontal connecting-processes or " arms," by 

 which the radial pillars are united at definite levels, 

 are very well developed in A, Tyrrellii, whereas 

 they are incompletely so in A. steUulatum. Hence 

 tangential sections of the former (PI. VIII. fig. 4) 

 show the " hexactinellid " structure characteristic of 

 the typical Actinostrojnce, while corresponding sections 

 of the latter want this stnicture and more closely 

 resemble similar sections in Clathrodictyon. 



h. The whorls of horizontal connecting-processes (which, 

 as seen in vertical sections, give rise to the " con- 

 centric laminas ") are developed at very short intervals, 

 being from i to jV millim. apart, and thus considerably 

 nearer to one another than are the radial pillars. 



