iambe.] CANADIAN PALEOZOIC CORALS. 93 



Tetradium fibratum, Safford. 

 Plate II., fig. 5. 



Tetradium fibratum, Safford. 1856. Am. Jour. Sci. and Arts, vol. XXII., p. 237. 

 Tetradium minus, Safford. 1856. Ibid, vol. XXII., p. 238. 

 Tetradium apertum, Safford. 1856. Ibid, vol. XXII., p. 238. 



Tetradium fibratum, Billings. 1863. Geology of Canada, pp. 136, 137, 139, fig. 71a, and 

 pp. 141, 149, 163, 177, 178, 185, 186, 194, 195, 218 and 938. 

 Nicholson. 1875. Palaeon. of Ont. p. 10. 

 Tetradium minus Nicholson. 1875. Palaeon. of Ont. p. 28. 



Tetradium Buronense, Foord, in parte. 1883. Contr. to Can. Cambro-sil. micro-pal. p. 

 25, pi. VII., figs. 16, lc, Id, le, (not la). 



Tetradium fibratum, Safford, the type of the genus, is described as 

 having a coralluin which is " massive, hemispherical, or flattened hemis- 

 pherical, composed of diverging tubes. Cell tubes four-sided with thin 

 and slightly rugose walls ; the four lamelloe distinct, nearly reaching the 

 centre of the tubes ; breadth of full-grown tubes usually about, or but 

 little more than half a line, varying occasionally from |rd to Jths of a 

 line. Transverse septa usually absent. A few have been seen in one 

 specimen, which were about twice the breadth of a tube apart." From 

 the upper half of the Lower Silurian rock of Middle Tennessee (Hudson 

 River). 



Tetradium minus was characterized by Safford as having smaller 

 corallites than T. fibratum, viz., from \\h. to ^rd of a line in breadth. 

 No other distinguishing characters were given. This fossil was collected 

 in the rocks of the upper division of the Lower Silurian series of Middle 

 Tennessee (Hudson River). 



Nicholson, in his Palaeontology of Ontario, p. 28, expresses a doubt as 

 to whether specimens of T. minus* from the Hudson River of the River 

 Credit, Ont., and at Manitouaning, Grand Manitoulin Island, are 

 really distinct from T. fibratum, as described by Safford. 



In Canada Tetradium is found at many localities in rocks of the Birds- 

 eye and Black River formation. The corallites in these specimens are 

 of rather unequal size, varying in specimens from different localities from 

 Jth to Jths of a line in width, and in individual specimens from Jrd to 

 f ths, from ^th to about J, and from Jth to Jrd of a line in breadth ; a 

 specimen from Gloucester Co., near Ottawa, has corallites with an 

 average width of J a line. The septa in some specimens reach nearly to 

 the centre of the corallites, in others they are not so highly developed. 

 The tabulae are seldom seen in longitudinal sections. 



*Nicholson in his Palaeozoic tabulate corals, p. 232, and in his manual of "Palaeontology, 

 vol. 1, p. 341, apparently regards T. minus as the type of the species. As T. fibratum 

 was described by Safford before his other species of Tetradium, it may be presumed that 

 the founder of the species looked upon T. fibratum as the type. 



