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



&^ 



Prof. SafFord further states that the isolated tubes are nearly 

 quadrangular, with more or less rounded angles, and with a 

 slight external longitudinal depression opposite to each of the 

 four septa ; the walls are more or less rugose ; and increase 

 is by fission of the old tubes. Only one specimen was seen in 

 which tabulffi could be detected ; and in this they were confined 

 to one end of the mass, and were distant from one another 

 about twice the width of the tubes. 



The genus Tetradium is regarded by Safford as intermediate 

 between the Favositidse and the Rugosa, the quadripartite' 

 character of the corallites placing it in the latter group. 



The late Mr. BilHngs (Geology of Canada, 1863, p. 139, 

 fig. 71) figures and quotes Tetradium fibratum^ SafFord, from 

 the Birdseye, Black-River, and Trenton Limestones of Canada; 

 but as no description is appended, it is difficult to form any 

 opinion as to the correctness of this identification, though the 

 few specimens we have from the Trenton Limestone would 

 apparently bear it out. 



Lastly, one of the present writers described Tetradium 

 minus, Safford, from the Hudson-river formation of Ohio and 

 Canada, and provisionally referred a coral from the Trenton 

 Limestone of Canada to T. Jlhratum, Safford (Nicholson, 

 Pal. Ohio, vol. ii. 1875, p. 222, and Rep. Pal. Ontario, 

 1875, pp. 10, 28). 



The above constitute all the notices of Tetradium which we 

 have been able to meet with, apart from mere enumerations of 

 species of the genus in lists of fossils. Though the characters 

 of the genus are extremely well marked, and the species are 

 far from uncommon in the Lower Silurian strata of North 

 America, no mention of the genus is made in the classical 

 works on fossil corals by Milne-Edwards and Haime (Pol. 

 Foss. des Terr. Pal. and Brit. Foss. Corals), nor by Fromen- 

 tel (Introduction a I'etude des Pol. Foss.). 



The remarks which we wish to make upon the genus may 

 be most fitly based upon a brief description of the American 

 T. minus, a species of which we have had the opportunity of 

 making a careful microscopical examination, and which may 

 be regarded as in all respects a characteristic representative 

 of the genus. 



Tetradium minus, Safford. 



Tetradium minus, SafFord, Amer. Joum. Sci. and Arts, ser. 2, vol, xxii. 

 p. 238 ; Nicholson, Palaeontology of Ohio, 1875, vol. ii. p. 222 ; 

 Nicholson, (Second) Report on the Pal. of Ontario, 1875, p. 28. 



Spec. char. Corallum massive, hemispherical, or amorphous, 

 often of large size, composed of closely approximated slender 



