234 TABULATE CORALS. 



the visceral cavities of the polypes are intersected by numer- 

 ous complete tabulae, which are straight and horizontal, and 

 entirely independent of each other (fig. 33, c). 



As regards the systematic position of Tctradium, the known 

 peculiarities in its structure seem to render three conclusions 

 more or less certain : (i) That Safford's conjecture as to the 

 Rugose affinities of the genus is entirely untenable ; (2) that the 

 genus has no strong points of relationship with Chcetetes, which 

 it most nearly resembles in general form and habit ; and (3) that 

 its true alliances are with Halysites and Heliolites. As regards 

 Halysites, the chief points of likeness are to be found in the 

 fact that both genera possess remarkably long tubular imper- 

 forate corallites, and in the fact that both Safford and Billings 

 have indicated that specimens of Tetradium sometimes assume 

 the otherwise unique habit of Halysites, the corallites being 

 united by their lateral margins in single intersecting series. I 

 have never myself been so fortunate as to meet with any speci- 

 mens exhibiting the peculiar mode of growth just alluded to, 

 but it is evident that whatever may be the relationship between 

 Tetradium and Halysites — and I do not doubt that there is a 

 relationship — the former genus is widely separated by its lamel- 

 lar, not spiniform septa, by the cruciform arrangement of these 

 structures, and by the total absence of any set of small zooids 

 coexisting with those of normal size. On the other hand, 

 Tetradium presents a very striking resemblance to Heliolites 

 in the form of the delicate lamellar septa, which have every 

 appearance of being "pseudo-septa" produced by inflection of 

 the wall. The absence, however, of small zooids, would of 

 itself be quite sufficient to prevent any actual reference of 

 Tetraditim to the Helioporidce, even if other differences (such 

 as in the number of the septa) did not exist as well. Upon the 

 whole, therefore, it seems best to regard Tetradium, Dana, as 

 the type of a special group of the Alcyonaria, allied to the 

 HalysitidcE and the Helioporidcs, and yet distinct from both of 

 these. 



