140 BRITISH STROMATOPOROIDS. 



have more the aspect of the genus Glathrodictyon than of Actinostroma. The 

 radial pillars are imperfectly or not at all recognisable in vertical sections, and the 

 hexactinellid network of well-preserved tangential sections is replaced by irregular 

 dark dots, only visible here and there, and partially connected with one another by 

 radiating " arms." My reasons for thinking that these may be only badly 

 preserved examples of the present species are : firstly, that even in well-preserved 

 specimens the radial pillars are not clearly recognisable in vertical sections if these 

 should be at all oblique; and secondly, that unquestionable examples of this species 

 sometimes fail to show the hexactinellid network in tangential sections in any parts 

 of the mass which are imperfectly preserved, but show instead a granular or dotted 

 aspect. The specimens to which I refer must, however, be studied in a more 

 complete series than I possess before it will be possible to assert positively that 

 they are referable to the present species. 



Distribution. — All the unquestionable examples of this species which I have 

 seen are from the Wenlock Limestone, occurring not very rarely at Ironbridge, 

 Much Wenlock, and Dudley. The Russian variety is from the Silurian Limestones 

 (zone of Pentamerus esthonus) of Kattentack, Esthonia. The doubtful specimens 

 above alluded to occur sometimes in the Wenlock Limestone ; but other similar 

 examples have been collected by Mrs. Robert Gray in the Ordovician Rocks 

 (" Balcletchie Conglomerate ") of Balcletchie, Girvan. 



6. Actinostroma stellulatum, Nidi. PI. XIV, figs. 1 — 8, and PI. XV. 



Stromatopoea concentrica, Maurer. Die Fauna der Kalke von AValdgirmes bei 



Giessen, p. 108, pi. ii, figs. 12 and 13. (Non 

 Stromatopora concentrica, Groldf.) 



Actinostroma stellulatum, Nich. Ann. Nat. Hist., ser. 5, vol. xvii, p. 231, pi. 



vi, figs. 8 and 9. 



The coenosteum in this species is sometimes laminar, with a basal epitheca, 

 sometimes massive. Massive specimens may be more or less spheroidal, and com- 

 posed simply of concentrically disposed strata; or they may be irregular in form, 

 and may be made up of a series of large-sized cylinders, each of which is composed 

 of concentrically arranged layers. The surfaces of successive strata are sometimes 

 smooth or gently undulated, but are at other times covered with low, rounded, 

 closely-approximated prominences or " mamelons." In well-preserved specimens 

 the free surface of the coenosteum exhibits the radiating astrorhizal grooves, and 

 the spaces between these are occupied by innumerable minute, rounded, or 

 elongated granules, representing the upper ends of the radial pillars. These 



