Chap. II. 



GRADATION AMONG ACALEPHS. 



121 



floating than active. A comparison between Porpita, VeleUa, and Physalia on one 

 side, and the Diphyidaj and Physophoridae on the other side, cannot fail to convince 

 any one Avho has seen any of these animals alive of the truth of this general 

 statement. When describing, in the sequel, the North American Hydroids in detail, 

 I shall have an opportunity of showing that the subdivisions founded upon the 

 differences here noticed among these animals are genuine sub-orders, and neither 

 orders nor fiimilies. 



Though I have not the remotest doubt that the Tabulata {Figs. 81 and 82) are 

 genuine Hydroids, I am not quite so confident that the Rugosa {Fig. 80) also belong 



Fig. 81. 



Fig. S2. 



GONIOPHTLLUM PYRAMIDALE, 



( Copied Jrom M.-Edwards tf- Haime.) 

 Upper figure, view from above; 

 lower figure, profile. Fossil of the 

 Silurian period. Comp. Lucemaria, 

 p. m. Figs. 75 and 76. 



oT' a^ 



MiLLEPORA ALCICOENIS, Lmk. 



Tr.insverse section of a branch of 



the Coral stock, magnified. 



a a Pits of the Hydroids, -with their suc- 

 cessive floors. It is very diHtcult to 

 obtain sections of the pits occupied 

 by the smaller Hydroids. 



Beaujiontia Egertoni, 



{Ccpiedymm M.-Edwards (^- Haime.) 

 Fossil of the Carboniferous period. 

 It resembles so closely the living Po- 

 cillopores, that it certainly belongs to 

 the same sub-order. 



to this class. I have not had sufficient opportunity of studying the Rugosa anew, 

 since I have known the acalephian affinities of the Tabulata, to feel justified in 

 expressing a decided opinion upon that point. I will therefore simply present my 

 reasons for believing that the Rugosa belong to the same class as the Tabulata. 

 The cavity occupied by the animal is divided by horizontal floors, evidently built 

 successively as in course of its growth the animal rose higher and higher, and 

 these floors are continuous from wall to wall across the whole width of the 

 cavity of the Coral; and wherever there exist radiating partitions, they rise only 

 from the surface of these floors, without extending through them to any other 

 floor above or below. No Coral known to be the soHd frame of a Polyp has 

 such a structure. On the contrary, in Polyparia the radiating partitions of the 

 individual cavities occupied by distinct animals extend uninterruptedly from top to 

 bottom of their cavities, and if there exist horizontal floors, these stretch only 

 across the intervals between two radiating partitions, and never across the whole 

 cavity occupied by the Polyp. The radiating partitions of the Rugosa, beside 

 being Hmited to successive floors, present another striking pecuharity, never observed 

 among the Polyps, — they are arranged in fours, or multiples of four. This quadri- 

 voL. III. 16 



