144 BRITISH STROMATOPOROIDS. 



Small specimens are often spheroidal or pyriform, and in some cases a wrinkled 

 basal epitheca is recognisable. The concentric lamina? are sometimes nearly 

 horizontal, sometimes simply curved to a greater or less extent, or sometimes 

 gently undulated. 



The surfaces of all the strata are covered with well-marked branching astro- 

 rhizae (Plate XVII, fig. 1), the centres of which are placed about 10 or 12 mm. 

 apart. The astrorhiza? do not appear to be arranged in superposed groups or 

 systems, and the ccenosteum is therefore not traversed by wall-less vertical canals 

 corresponding with the axes of such systems. For the same reason, the surface 

 does not exhibit definite " mamelons," though small rounded eminences are some- 

 times irregularly developed. 



As regards internal structure, the ccenosteum is composed of exceedingly 

 delicate and close-set radial pillars, of which from twelve to fifteen may occupy 

 the space of 1 mm. The radial pillars are " continuous," and they are not inter- 

 rupted in their course by the concentric lines of growth which intersect the 

 skeleton. The radial pillars (Plate XVII, fig. 4) give out exceedingly delicate 

 horizontal connecting processes or " arms," which give rise, as seen in tangential 

 sections (Plate XVII, figs. 2 and 5), to a correspondingly delicate " hexactinellid " 

 structure. About twenty interlaminar spaces occupy the space of 1 mm., but the 

 proper concentric lamina? are irregular and often more or less broken or reticulate 

 (as they are in A. intertextum, Nich.). 



A constant and exceedingly characteristic feature of vertical sections (Plate 

 XVII, figs. 3, 4, and 7) is that the entire ccenosteum is divided by well-marked 

 and quite definite concentric lines, which have nothing to do with the ordinary 

 concentric lamina? nor with latilarninae. These concentric " lines of growth," as 

 they may be termed, are placed at distances of from x^th mm. to 1 or 2 mm. 

 apart ; and they are usually arranged in groups of close-set lines separated by 

 wider bauds in which these lines are few or wanting. 



Obs. — When I first fully described this remarkable species (' Ann. Nat. Hist.,' 

 ser. 5, vol. xvii, p. 229, 1886), I was not absolutely clear as to its being truly referable 

 to the genus Actinostroma. This uncertainty arose from the fact that von Rosen's 

 original specimens, as also the majority of all specimens hitherto examined, are in 

 such a highly crystalline condition that their minute internal structure is hardly to be 

 deciphered. I have, however, now examined specimens, from the Silurian Rocks 

 of Britain, in which the internal structure is very fairly preserved, and these leave 

 no doubt as to the fact that the species, as I conjectured, is properly referable to 

 the genus Actinostroma. The species of this genus, with which A. astroites is 

 most nearly related in general structure, is unquestionably A. intertextum, Nich. 

 The present species is, however, distinguished from this, as from all the other 

 known forms of Actinostroma, by the extraordinary delicacy of its skeletal tissue, 



