the Genus Tetradium, Dana. 165 



Characters and Affinities of the Genus Tetradium. 



From a consideration of the characters of T. minusj as 

 above discussed, the following diagnosis of the genus Tetradium 

 may be framed : — 



Corallum massive, subhemispheric or irregular in form, 

 composed of long prismatic corallites, in close contact, but 

 not amalgamated by their walls, and not aiTanged in super- 

 imposed strata in the typical forms. No mural pores ; septa 

 distinct, few in number, typically four, short, not reaching the 

 centre of the visceral chamber, seeming as if formed by in- 

 flections of the wall. Calices generally petaloid, as are the 

 corallites in transverse section. Tabulae numerous, complete. 

 Increase by fission of the old tubes. 



It will be seen from the above definition that Tetradium 

 is clearly what is generally understood as a " Tabulate " 

 coral, and that it is not referable to the Rugosa. Its septa, 

 though quite distinct, are hardly more developed than can be 

 observed in well-preserved specimens of Ilalysites or Syrin- 

 gopora^ or even Favosites itself, and are certainly no moi'e so 

 than in most of the Alcyonarian group of the Heliolitidte. 

 Moreover the septa are quite unlike those of the Rugose corals 

 in general, and, from their distinct production by inflection of 

 the walls of the corallites, they assume the character of 

 " pseudo-septa." The quadripartite character of the tubes 

 can also not be relied upon as allying the genus with the 

 Rugosa, since the septa, though generally four in number, 

 vary from one to as many as five. 



Amongst the other groups of corals which have been gene- 

 rally referred to the miscellaneous division of the " Tabulata," 

 it is difficult to find one to which Tetradium could be re- 

 ferred with entire propriety. From the Alcyonarian family 

 of the Heliolitidse the present genus is distinguished by the 

 fact that the corallites are all of one size, and there is no tubu- 

 lar " coenenchyma." With the Halysitidai the genus has 

 some decided affinities, which are increased by Prof. Safford's 

 observation that the corallum sometimes resembles that of 

 Halysites or of Syringopora in form. We have not, however, 

 noticed this mode of growth in any of the specimens which 

 have been examined by us ; and the corallum in general is 

 quite similar in form to that of the massive species of Favo- 

 sites or ChcBtetes. Under any circumstances, should the genus 

 be ultimately referred to this family, it will be in the imme- 

 diate neighbourhood of Halysites itself that it must find its 

 place ; and we are disposed to think that our present know- 

 ledge will perhaps allow of no better provisional arrangement. 



