166 Dr. H. A. Nicholson and Mr. R. Etheridge on 



From the Favositidse, at any rate, Tetradium is sharply sepa- 

 rated by the imperforate walls of its corallites ; and the only 

 other group that needs consideration in this connexion is that 

 of the Chgetetidae. In general form and appearance there is 

 the closest possible resemblance between the present genus and 

 some of the massive forms of Chcetetes or Monticulipora ; but 

 the peculiar septa of the former are quite sufficient to distin- 

 guish them. The only species of Chcetetes which possess any 

 structures at all comparable with the septa of Tetradium are 

 C. radians^ C. [Alveolites) septosits, and C. {Alveolites) de- 

 pressus. The so-called septal processes of these forms, how- 

 ever, are not present in all the corallites of a given corallum, 

 they are variable in number and development, and, as we have 

 elsewhere given reasons for believing, they appear rather to 

 be caused by the fission of the old tubes than to be of the 

 nature of septa. 



Upon the whole, therefore, it is perhaps safest to regard 

 Tetradium as an ally of Halysites, with some affinities to Ghce- 

 tetes, and thus forming a connecting link between the families 

 of the Halysitidse and ChsetetidsB. 



Tetradiiim Peachii, Nich. & Eth., jun. 

 Alveolites ? sp., R. Etli, jun. Proc. Rov. Phys. Soc. Edinl). 1874-75, 



p. 51. 

 Alveoliteft? Peachii, R. Eth. jun. (MS.), Coll. Geol. Survey of Scotland, 



Mus. Science and Art, Edinb. 



Spec. char. Corallum massive, exceedingly dense and com- 

 pact, composed of excessively minute, closely approximated 

 corallites, about a thirty-fifth of a line in diameter. The 

 corallites have an undulating course, and are sometimes dis- 

 posed in superimposed layers, or aiTanged concentrically roimd 

 minor centres. Corallites thick-walled, iiTCgularly circular 

 or oval in transverse section, with a few (three or four ?) short 

 septa, which are often thickened at their bases. Tabulae nu- 

 merous and complete. Corallum perforated by iiTcgular tubes 

 (water-canals ?) from a fortieth to a fiftieth of an inch in dia- 

 meter or less. 



Obs. We are only acquainted with this very remarkable 

 coral as occurring in the pebbles of Silurian limestone which 

 are found in the Old Red conglomerate at Habbies Howe, 

 in the Pentland Hills. That these pebbles are of Upper- 

 Silurian age cannot be doubted, as they contain Halysites 

 catenidarius, Heliolites^ Phacojps^ and other characteristic fossils 

 of this period, though no similar limestone has hitherto been 

 recognized in the Upper Silurian formation of Scotland. The 



