68 BRITISH STROMATOPOROIDS. 



A similar resemblance, though not so striking, may be found if we take one 

 of those HydracUnice which produce a calcareous coenosteum. The only one 

 of these which I have been able to examine by means of thin sections is the 

 Hydractinia circumvestiens 1 of the Red Crag and Coralline Crag of Suffolk. In 

 this interesting species the skeleton is calcareous, and forms crusts of considerable 

 thickness growing upon species of Trophon or other Gasteropodous shells. Viewed 

 in thin sections by transmitted light, the skeleton appears to be composed of irre- 

 gular calcareous grains closely fitted together (Plate VI, fig. 13) ; but I am not 

 able to say whether or not this is the result of secondary alteration. A rough ver- 

 tical fracture (Plate VI, fig. 7) shows that the skeleton is traversed by numerous 

 irregular vermiculate tubules, which are approximately vertical, and run parallel to 

 one another at little distances. These vertical tubules are interrupted at intervals 

 by irregular chamberlets, placed in roughly horizontal lines, so as to give rise to 

 imperfect " interlaminar spaces," and to confer upon the skeleton an indistinct 

 lamination. The vertical tubules appear to have lodged the polypites, or to have 

 given passage to stolons of the ccenosarc, and they either terminate in the 

 chamberlets above mentioned (which at one time formed successively the surface 

 of the colony), or they terminate above in minute round apertures on the free 

 surface (Plate VI, figs. 8, 9, and 10). Thin vertical sections (Plate VI, fig, 11) 

 show much the same phenomena as rough fractures ; but the skeleton is now seen 

 to be traversed at intervals by a series of vertical " radial pillars " of compara- 

 tively large size, anci of a more or less open and cribriform texture in their central 

 axes. These large pillars are also recognisable in tangential sections (Plate 

 VI, fig. 12), and they terminate on the surface in prominent round tubercles, 

 which, in some instances at any rate, seem to be furnished with distinct central 

 perforations (Plate VI, figs. 9 and 10). In addition to these large and seemingly 

 hollow pillars, the surface shows numerous small imperforate tubercles, together 

 with well-developed branching astrorhizal grooves (Plate VI, figs. 9 and 10). 



From the above sketch of the structure of the skeleton in Hydractinia echinata, 

 Flem., and H. circumvestiens, "Wood, it will be seen that these types exhibit 

 marked points of likeness to certain forms of the Stromatoporoids, with, at the 

 same time, equally marked points of dissimilarity. It will also be seen that so far 

 as H. echinata, Flem., is concerned, it is only with a particular section of the 

 Stromatoporoids that the likeness is at all very conspicuous. The section to 

 which I refer is that comprising the genera Actinostroma and Clathrodictyon, and 

 their allies — what may be called the section of the " Hydractinioid " Stromatopo- 



1 Hydractinia circumvestiens was described by Searles "Wood under the name of Alcyonidium circum- 

 vestiens (" Catalogue of Zoophytes from the Crag," 'Ann. and Mag. Nat. Hist.,' ser. i, vol. xiii, 1844). 

 I should be disposed to think that it is really identical with the form described at a later date by Dr. 

 Allman, under the name of H. plioccena (' Geol. Mag.,' 1S72, p. 337) ; but I have not had the 

 opportunity of examining the latter. 



