236 PKOr. H. A. NICHOLSON AND DU. J. MUEIE ON THE 



cially examined oar numerous thiu sections of various Stromato- 

 jDoroids, with a view of collecting all the evidence wliicli these 

 might offer in support or disproof of this view. IS'ot having had 

 the opportunity of thorovighly examining any calcareous sj^ecies 

 of Hydractinlaj we should feel it presumptuous to express a final 

 opinion on this question; but we submit the following consider- 

 ations as, in our judgment, warranting the belief that at present 

 there is insufficient evidence to connect the Stromatoporoids with 

 Hydractinia, though such a connexion may subsequently be esta- 

 blished. 



First. As to general form and habit, few StromatojDoroids could 

 be properly said to be incrusting, as compared with a Sydractinia, 

 since they very rarely form thin crusts attached by the whole of 

 the lower surface to foreign bodies. On the contrary, they either 

 form massive aggregates, like those of many Corals, which may 

 be attached by one point to a foreign body, or may have grown 

 round such as a nucleus ; or, in many cases, they form extended 

 expansions, quite -like those of many Corals (such as species of 

 JFistiilipora, Favosites, &c.), attached by a comparatively limited 

 point to some foreign object, and having almost the whole of the 

 lower surface free and covered with a well-developed calcareous 

 striated membrane or "epitheca." . 



* Secondly. The resemblance between the minute structure of 

 the crust of Hydractinia and that of the typical Stromatoporoids 

 is, in our opinion, a purely general one, and is not nearly so close 

 as the resemblance between the Stromatoporoids and certain of 

 the perforate Corals or such Hydrozoans as Millepora. This is 

 particularly well seen by a comparison of magnified thin vertical 

 sections of examples of these different groups. 



Thirdly. The resemblance of the tuberculated superficial layer 

 of certain Stromatoporoids to the upper surface of the crust of 

 Hydractinia is also, in our opinion, a superficial one; whilst mauy 

 Stromatoporoids, precisely agreeing with the former in minute 

 structure and in the general arrangement of their parts, have per- 

 fectly smooth surfaces. 



Fourthly. Many Stromatoporoids show very unmistakably two 

 sets of apertures, one large and the other small, upon the surface, 

 the large openings being often placed upon the end of rounded 

 or conical eminences, and being often extremely regular in their 

 arrangement. These phenomena cannot be paralleled by any 

 thing exhibited by Hydractinia, so far as we are aware. 



