SYSTEMATIC STRUCTURE AND AFFINITIES. 71 



(" gastropores " and " dactylopores "j, while their skeleton-fibre has a peculiar 

 minutely porous structure. Moreover, in various forms of Stromatopora we can 

 demonstrate, more or less clearly, the existence of the " radial pillars," which 

 are such characteristic structures in the Stromatoporoids generally. 



If, lastly, we turn to the group of the Stylasteridce, as exemplified by such a 

 type as Distichopora, Lam., we find, again, certain likenesses to the Stromatopo- 

 roids as well as certain marked differences. In Distichopora the zooidal tubes are 

 divisible, as they are in Mill&pora, into two distinct series (" gastropores " and 

 " dactylopores ") which occupy definite tracts of the coenosteum. The general 

 skeleton is composed of dense calcareous tissue, excavated in every direction by 

 branched and anastomosing, microscopic coenosarcal canals (Plate IV, fig. 4, and 

 Plate IX, fig. 5). Lastly, in the species I have examined, I find that the pore-tubes 

 are traversed by sparsely-developed, complete, transverse partitions or " tabulae," 

 which seem, however, to be confined to the deeper portions of the zooidal tubes, 

 and to disappear as the surface is approached. 



There are, unquestionably, strong points of resemblance between such a Stylas- 

 terid as Distichopora and such Stromatoporoids as Stachyodes, Barg. In this latter 

 genus (Plate VIII, figs. 9 — 14), the cosnosteum is dendroid; there are zooidal 

 tubes, which open by definite apertures upon the surface (though only doubtfully 

 divisible into two distinct series) ; and the general tissue of the skeleton is traversed 

 by innumerable microscopic tubuli. The genus Stachyodes, however, is an aberrant 

 type, and with regard to the more normal Stromatoporoids we may in the mean- 

 while leave the Stylasterids comparatively out of view, as apparently further 

 removed from these ancient Hydrozoa than either the Hydractinio3 or the 

 Milleporce. 



Upon the whole, therefore, it would appear that certain of the Stromatoporoids 

 (such as Actino stroma, Nich., and Labechia, E. and H.) have a skeleton in many 

 respects resembling that of the Hydractiniidce ; while others (such as Stromatopora, 

 Goldf.) possess hard structures which are more closely comparable with the ccenos- 

 teum of Millepora. As, however, these two groups of Stromatoporoids are linked 

 together by various intermediate forms {Clathrodictyon, StromatoporeUa, &c), and 

 as the natural series thus constituted possesses an aggregate of characters distinct 

 from those of either the Hydractiniidai or the Milleporidce, it would not accord 

 with the principles of sound classification to merge the former in either of the last- 

 named divisions of the Hydrozoa. I shall, therefore, retain the name of Stromato- 

 poroidea for the whole group of organisms now under consideration, regarding 

 them as a peculiar division of the Hydrozoa, with affinities to the Hydractiniidce on 

 the one side and to the Hydrocorallince on the other side. The propriety of thus 

 keeping the Stromatoporoids as a separate group is the more evident when it is 

 remembered that our knowledge of these singular organisms is necessarily derived 



