ACTINOSTROMA CLATHRATUM. 133 



however, that precise measurements of this kind possess but a limited and general 

 value, even individual specimens commonly showing more or less variability as 

 regards the closeness of the pillars and laminse. Moreover, specimens collected 

 from distant localities very generally show slight but apparently constant differences 

 in this respect. 



A. clathratum is liable to considerable variation, and among the forms which 

 I am disposed to include under this title are three recognisable types, two of 

 which are so far distinct from one another that some paleontologists would probably 

 consider them to be separate species. 



In the first of these varietal forms — which I regard as the type of the species 

 — the radial pillars are very regularly developed, the distances between them 

 being approximately uniform in a given specimen (generally about y or ^ mm.). 

 The pillars are, further, remarkably parallel to one another, making due allowance 

 for the intercalation of new ones as the surface is approached, and in cross-sections 

 they usually have a well-marked round shape (Plate I, figs. 11 — 13). Astrorhizse 

 may be wholly wanting, but there are usually small astrorhizse to be detected in 

 tangential sections, though the development of these is always feeble. Thi& 

 form of A. clathratum is the one which occurs commonly in the Middle Devonian 

 of Germany (Gerolstein, Hebborn, &c), and it likewise occurs in Devonshire, 

 though it is not the most abundant form in the latter region. 



In the second of the varietal forms in question, the radial pillars are compara- 

 tively irregular in their development, and vary considerably, even in the same 

 specimen, as regards their distances apart, while they are often variously bent or 

 sloped; and they show a want of general parallelism to each other (Plate XII, fig. 

 3). In transverse sections (Plate XII, fig. 2), the pillars are often more or less 

 angular, rather than strictly round. Upon the whole, also, the pillars are mostly 

 farther apart than in the preceding variety, being usually separated by interspaces 

 of about ^ or ^ mm. Lastly, astrorhizae are comparatively well developed, and are 

 not only numerous but comparatively large. This form occurs commonly in the 

 Devonian Rocks of Devonshire, being particularly abundant at Dartington. If it 

 should be thought necessary to distinguish this variety by a special name, I should 

 propose to call it A. irregulare. 



Finally, there exists a third varietal form which is intermediate in its characters 

 between the two previously described. The form in question agrees with the 

 normal form of the species in having stout round pillars (Plate XIII, fig. 1), but it 

 resembles the last-named variety in having the pillars irregularly developed, and 

 incompletely parallel (Plate XIII, fig. 2). This variety occurs comparatively rarely 

 in the Devonian Limestones of Devonshire. 



As compared with the other species of the genus, Actinostroma clathratum, 

 Nich., is most nearly related to A. verrucosum, Goldf., with which it closely agrees 



