HALYSITIDJE AND TETRADIID^. 229 



interstitial tubes — so far as my observations go — seem to be 

 wanting ; but I have never failed to recognise their existence 

 in H. catemdaria, Linn. ; and they are especially well developed 

 in all specimens of H. agglomerata, Hall, which I have ex- 

 amined. 



With regard to the septa of Haly sites, the condition of parts 

 varies greatly in different forms of the group. In those forms, 

 namely, which are usually referred to H. catenularia, Linn., I 

 have never succeeded in recognising in microscopic sections 

 any traces whatever of septa, except that the lateral walls of 

 the intermediate or small corallites often carry small projec- 

 tions apparently of a septal nature (PI. XL, fig. i). On the 

 other hand, in the forms which are usually known as H. 

 escharoides, Lam., there are always well -developed spiniform 

 septa, of exactly the same type as in the Favositidcs and 

 SyringopoiHdiS. These septa are arranged in vertical rows, 

 the number of which seems to be constantly twelve in each 

 corallite. 



I do not intend here to give any description of any of the 

 species of Halysites, especially as I am unable to make up my 

 mind as to the true relations of H. cafenuiaria and H. eschar- 

 oides to one another. Till now I have ventured to differ from 

 such distino-uished authorities as Milne-Edwards and Haime, 

 and have always considered these as mere varietal forms, since 

 I have looked upon the sole alleged differential characters — 

 viz., the size of the tubes and the dimensions of the meshes of 

 the corallum — as purely varietal characters. By means of micro- 

 scopic sections, I have satisfied myself — so far as my material 

 goes — that the form known as H. escharoides, Lam., is distin- 

 guished from the typical H. catenularia, Linn., not only by 

 the superficial characters just mentioned, but also by the con- 

 stant possession of spiniform septa, and the apparently constant 

 absence of small tubes between the larger one. As these dif- 



access, but this observer does not seem to have recognised the true nature of the 

 interstitial tubes. To Dr Lindstrom, therefore, is due the credit of having pointed 

 out that the corallum oi Haly sites, as of Heliolites, consists of two distinct sets of 

 zooids (Ofversigt af Kongl. Vetenskaps Akad. ForhandL, 1873). 



