imperfectly-known Species of Stromatoporoids. 321 



In microscopic structure A. Whiteavesii is atypical Actino- 

 stroma, the ccenosteum consisting of parallel radial pillars 

 united at corresponding levels by whorls of horizontal con- 

 necting-processes or " arms." As seen in vertical sections 

 (PI. IX. fig. 4) the radial pillars are of moderate thickness, 

 about ten occupying the space of 2 millim. measured trans- 

 versely, -while the whorls of connecting-processes (forming 

 the " concentric laminae ") are decidedly more distant, about 

 seven only of these structures being present in the same space 

 measured vertically. 



In tangential sections (PI. IX. fig. 3) the rounded ends of 

 the transversely-divided radial pillars are seen to be united by 

 very regularly developed whorls of connecting-processes, the 

 thickness of which is very nearly equal to the diameter of the 

 radial pillars themselves. The result of this is that tan- 

 gential sections coinciding with the plane of the laminae show 

 a characteristic areolated structure, which is not at all unlike 

 that exhibited by corresponding sections of some Monticu- 

 liporoids. 



Obs. In its general aspect and its surface-characters the 

 present species resembles A. expansum, Hall and Whitfield, 

 the most obvious difference being that the low and rounded 

 eminences characterizing the surface of the laminae in both 

 species are smaller and nearer together in A. Whiteavesii than 

 in the case of the latter. A. Whiteavesii also more or less nearly 

 resembles the Stromatoporoids which Quenstedt has figured 

 from the Devonian rocks of North America under the names 

 of Stromatopora pustuhfera and S. monticulifera (' Petrefak- 

 tenkunde Deutschlands : Schwamme,' Taf. cxlii.). As I have, 

 however, elsewhere pointed out, mere external features, taken 

 by themselves, are as a rule wholly unreliable in determining 

 the affinities of the Stromatoporoids, and only become of value 

 when taken along with the characters derived from the internal 

 structure of the skeleton. Quenstedt's figures and descrip- 

 tions of the forms to which he has given the above names are, 

 however, unfortunately of little use except to give an idea of 

 external features, and leave us almost entirely ignorant of the 

 real structure of the ccenosteum. It is therefore impossible 

 to be certain as to even the generic position of the forms in 

 question. Both forms, however, are stated to possess well- 

 developed astrorhizae of the ordinary type, and are figured 

 with such ; and it may therefore be taken as tolerably certain 

 that neither A. expansion, Hall and Whitf., nor A. Whiteavesii, 

 Nich., can be identical with either of Quenstedt's types. 



Though, as just stated, A. Whiteavesii has a general resem- 

 blance to A. expansunif Hall and Whitf., it is at once distin- 



