Structure of Stromatopora. 259 



lamina in which it is developed I am not prepared to state ; 

 but it often appears to descend through several layers in the 

 mass when this is merely owing to the undulation of the 

 layer; while in flat species a horizontal shave more or less, as 

 before stated, may bring it into view or take it away alto- 

 gether, showing that in this instance it is confined to the 

 layer. Still, as the cavities of the ccenosarc become continuous 

 by extension upwards, so the branches of the flexuous ray of 

 the stellate venation may be more or less extended vertically. 

 Hence also, as the form presented by the free surface of 

 Stromatopora cannot be generic but must be specific, so this 

 must be given, together with the other peculiarities of struc- 

 ture, in the descriptions of the respective species. 



We have now to consider the import of the parts that 

 have been enumerated, and see if this can be elucidated by 

 reference to existing species. 



It is true that no species of Stromatopora is known to exist 

 at the present day; but still there may be organisms of a like 

 nature that reflect back a light which may lead us to a right 

 understanding of what Stromatopora was. 



That Millepora alcicornis, although not growing into such 

 thick masses as Stromatopora, does produce a corallum which 

 is composed of a curvilinear ccenenchyma, increasing by lami- 

 nation, and spreading over or enveloping every hard object 

 with which it comes into contact, like Stromatopora, is incon- 

 testable. 



That the fossil species, viz. Hydractinia pliocena (origi- 

 nally and significantly called Stromatopora incrustans by 

 Goldfuss, as before stated), was produced by a like organism 

 to that of Millepora is equally true. 



And, lastly, we have species of Hydractinioz at the present 

 day producing respectively horny and calcareous polyparies 

 analogous to these coralla. 



Nor does any one doubt that all these were produced by 

 Hydroid Polyps issuing from holes smaller even than those 

 indicated by the triangular spaces in the rectilinear structure 

 of Stromatopora. In short, when we look at a whelk-shell 

 covered with living Hydractinia echinata in its natural ele- 

 ment, or after having suddenly been plunged from it into 

 spirit and water for preservation, we do not wonder that such 

 an exuberant growth of polyps of all sizes, such as must also 

 be present in Millepora alcicornis, could easily produce the 

 coralla of either Hydractinia pliocena or any of the Stroma- 

 toporce. 



So far, then, we can account for the corallum of Stroma- 

 topora and the animal which produced it. 



