SKETCH-CLASSIFICATION. 73 



which have still to be examined by modern methods, the following classification 

 must be regarded as largely tentative, though I think it will bo found to indicate 

 the lines upon which any future classification must be based. Students of recent 

 forms may be inclined to consider the number of families proposed as out of all 

 proportion to the number of genera. It should be remembered in this connection, 

 however, that many more generic types almost certainly remain to be yet discovered, 

 and that the forms at present known are in all probability only the widely separ- 

 ated links of a great series of extinct Hydrozoa, of which our knowledge is at 

 present very imperfect. I shall subsequently discuss the characters of the families 

 and genera at some length ; but it may be as well, at the risk of some repetition, 

 to subjoin here a brief tabular view of the classification which I have ventured to 

 suggest. 



Order— STROMATOPOROIDEA, Nidi, and Mur. 



Hydroid Zoophytes producing a calcareous ccenosteum, which may be encrust- 

 ing or dendroid; but which is most commonly laminar or massive, with a basal 

 epitheca, and a comparatively small peduncle of attachment. Ccenosteum com- 

 posed essentially of two sets of elements, viz. : (1) hollow or solid calcareous rods, 

 or pillars, which are " radial" in position, or are vertical to the general surface; 

 and (2) hollow or solid calcareous fibres or plates, which are in the main rect- 

 angular to the preceding, or "tangential" to the general surface, and which are 

 developed at more or less definite intervals, thus giving rise to a series of horizon- 

 tal " laminae." The radial pillars may be much modified, or even partially sup- 

 pressed as definite structures. Very generally the horizontal fibres are more or 

 less closely united with one another and with the radial pillars, and thus give rise 

 to a reticulated skeleton. 



The skeleton-fibre may be apparently solid, but in other cases is minutely porous 

 or tubulated. 



Definite zooidal tubes may be present or absent. When present, they are 

 usually " tabulate," and appear in general to be approximately similar to one another 

 in size and internal structure. 



" Astrorhizal canals " may be present, or absent. [No account is taken of the 

 so-called " Caunoporce " in the above definition, as the nature of the fossils so named 

 will be dealt with separately.] 



