2 40 TABULATE CORALS. 



evident that Dr Lindstrom had much ground for his suggestion 

 (Ann. Nat. Hist., sen 4, vol. xviii. p. 13) that Thecia is closely 

 allied to the HeliolitidcE. As has been already pointed out, 

 however, there exist certain very important differences between 

 Thecia and Heliolites, which are of too fundamental a nature to 

 permit of our associating these genera in a single family. Of 

 these differences, the most important is the presence in the 

 former genus of distinct and well-developed horizontal canals, 

 which traverse the interstitial tissue, and place the visceral 

 cavities of neighbouring polypes in direct communication This 

 feature is one in which the genus approaches the Perforate 

 corals in general, and the FavositidcB in particular ; but the 

 latter are fundamentally distinguished by the total absence of 

 anything analogous to the interstitial tubuli of Thecia. Upon 

 the whole, therefore, the best course to follow is to place Thecia 

 in a distinct family — ThecidcSi E. and H. — which will occupy a 

 position in the Alcyonaria close to the HelioporidcB ; and in so 

 doing we may remember that though the horizontal canals 

 above spoken of recall to our minds the " mural pores " of 

 the FazwsitidcB and the hollow " connecting-processes " of the 

 Syringoporid^, the existence of a direct connection between 

 the visceral cavities of neighbouring polypes is a common 

 feature in the Alcyonarian Zoophytes. Before leaving the 

 subject of the affinities of the genus Thecia, a few words may 

 be said as to the views which Dr Rominger has expressed upon 

 the same point (Foss. Cor. of Michigan, p. 65). According to 

 this observer, Thecia has "the general structure oi Favosites ;" 

 the walls of the corallites are thick, and are bounded by "de- 

 fined polygonal outlines," but occasionally " the walls do not 

 exceed in thickness those of an ordinary Favosites ; " the septa 

 are spiniform, and extend nearly to the centre of the visceral 

 chamber ; and " large and abundant " lateral pores {i.e., mural 

 pores) are present. The figures given by Dr Rominger, beauti- 

 ful as they are, being heliotypes, present the specimens of the 

 natural size, and give one no clue as to their real internal struc- 

 ture. Not having seen Dr Rominger's specimens, it would be 



