52 BRITISH STROMATOPOROIDS. 



either a very young specimen of L. conferta, Lonsd., or perhaps a new species. In 

 this specimen the coenosteum is a thin discoid expansion, covered below by a 

 delicate striated epitheca, which bears superiorly a single layer of blunt imperforate 

 tubercles ; there being no traces of superficial apertures, nor any room for the 

 existence of vertical tubes. In the adult Ldbechia conferta, Lonsd., it seems prob- 

 able that the zooids were likewise given off from the surface-layer of the ccenosarc ; 

 the principal change effected in the course of growth being, that as the radial pillars 

 grew upwards the spaces between them became divided into cellular compartments 

 by the development of curved calcareous plates. In the adult Hydractinia echinata, 

 on the other hand, the successively formed layers of the coenosteum are not abso- 

 lutely imperforate but are traversed by numerous minute pores, by which the 

 entire ccenosarc is kept in organic connection, and from the last series of which 

 the zooids are emitted. 



In Actinostroma clathratum, Nich., and its immediate allies, the concentric 

 lamina? are, as has been already pointed out, minutely porous (Plate I, figs. 8 and 

 11). They are composed of calcareous filaments so interlaced as to leave between 

 them innumerable minute apertures, which pass through the lamina? and place 

 successive interlaminar spaces in direct communication. The existence of such 

 pores can generally be made out by a simple examination of the surface with a 

 hand lens, and always by means of properly prepared thin sections taken parallel to 

 the lamina?. In Actinostroma clathratum itself these pores are simply the wide 

 angular meshes formed by the inosculation of the horizontal arms which are thrown 

 out from the pillars ; and it seems certain that their function must have been that 

 of transmitting stolons by which the ccenosarc in successive interlaminar spaces 

 was bound together. "We may also reasonably suppose that in the last formed and 

 most superficial concentric lamina the pores would correspond with the points at 

 which the separate zooids were budded off, and that these openings therefore 

 represent zooidal tubes. 



As to whether or not dimorphism of the colony occurs in any of the Stromato- 

 poroids, it is not easy to speak with certainty. Mr. Champernowne has been 

 good enough to furnish me with examples of a species of Stromatopora — apparently 

 an undescribed form — in which scattered among the ordinary tubes are tolerably 

 regularly placed tubes of larger size, both sets of tubes being tabulate. This can 

 hardly be interpreted as other than a case of dimorphism ; but it appears to be an 

 exceptional case, and I have not been able in the other species of Stromatopora to 

 recognise any marked or constant differences between different zooidal tubes. 

 When we consider, however, how slight, comparatively speaking, are the differences 

 between the gastropores and the dactylopores of the coenosteum of Millepora, it 

 may be conjectured that dimorphism may well have existed generally in the genus 

 Stromatopora, without our being able to demonstrate this from the hard parts alone. 



