230 TABULATE CORALS. 



ferential characters are fundamental ones, It may seem ridicu- 

 lous to doubt any longer the specific distinctness of H, escJiar- 

 oides and H. catemdaria. The two forms, however, are so re- 

 markably similar in most respects, are so variable in external 

 characters, and are so constantly associated together, that I am 

 still almost disposed to conjecture that they are the different 

 conditions of a dimorphic species, and that their differences 

 are due to something else than specific distinctness. 



As regards the affinities and zoological position of Halysites, 

 the above account of its structure leaves little doubt that the 

 genus is more nearly allied to the Heliolitidce than to any 

 other. Halysites agrees with Heliolites and the recent Helio- 

 pora in possessing (usually) two distinct sets of corallites, large 

 and small, the two being distinguished further by the arrange- 

 ment of their tabulse ; and there can be no reasonable doubt 

 that this indicates that each colony of the typical H. catemdaria, 

 Linn., consisted of two structurally and functionally distinct sets 

 of zooids. On the other hand, I cannot accept the view of Dr 

 Lindstrom (Ann. Nat. Hist., sen iv., vol. xviii. p. 13), who 

 actually places Halysites among the HeliolitidcB. It appears 

 to me, on the contrary, to form a quite special group, distin- 

 guished from Heliolites and its allies, not only by the unique 

 form of the corallum, but also by the great reduction, or, it 

 may be, total absence, of the smaller zooids, by the frequent 

 absence of septa, and by the fact that the septa (when present) 

 are spiniform, and present no resemblance to the "pseudo-septa" 

 of the HelioporidcE. If the view that Halysites is nearly allied 

 to the Helioporidce be accepted, then it follows that the family 

 of the HalysitidcE will have to be considered as a distinct and 

 ancient group of the Alcyonaria ; but it will still remain certain 

 that no true relationships have been shown to exist between 

 Halysites and the Ttibiporidcs, with which family the genus has 

 often been associated. 



The geological range of Halysites, so far as known, is an 

 extremely limited one, the first representatives of the genus 

 appearing in the later portion of the Lower Silurian period. 



