HALYSITIDyE AND TETRADIID^. 231 



while the maximum development of the genus is attained in 

 the Upper Silurian, and no Devonian species has been as yet 

 detected. 



TETRADIID^. 



This group includes only the Silurian genus Tetradmin, 

 Dana, and its probable systematic place will be best discussed 

 after some description of the genus has been given. The 

 fullest account of the genus which has yet been published is 

 one which was given by Mr R. Etheridge, jun., and myself 

 (Ann. and Mag. Nat. Hist., sen iv., vol. xx. p. 162, 1877) ; and 

 the following brief analysis of the chief characters of Tetradium 

 is for the most part extracted from the memoir just referred to. 



Genus Tetradium, Dana, 1846. 

 (Wilkes's Expl. Exped. Zoophytes, p. 701.) 



Gen. Char. — Corallum massive, composed of long prismatic 

 and closely contiguous corallites, the walls of which are not 

 pierced by mural pores or other foramina. The septa are 

 distinct, few in number, most typically four, short, not reach- 

 ing the centre of the visceral chamber, and seeming as if 

 formed by inflections of the wall. Calices generally petaloid, 

 as are the corallites in transverse section. Tabulae numerous, 

 complete. Increase apparently by fission of the old tubes. 



Obs. — The genus Teh^adium was founded by Professor Dana 

 for the reception of a fossil of uncertain locality in the collection 

 of Yale College, New Haven ; and the following characters were 

 ascribed to it : " Coralla massive, consisting of four-sided tubes 

 and cells, with very thin septa or parletes ; cells stellate, with 

 four narrow laminae." 



At a subsequent date. Professor J. M. Safford described four 

 species of corals from the Lower Silurian Rocks, which he re- 

 ferred to this genus (Amer. Journ. Sci. and Arts, ser. 2, vol. 

 xxii. p. 236) ; and he supplemented Dana's description with 



