24 GEOLOGICAL AND NATURAL HISTORY SURVEY OF CANADA. 



Arthroncma ienue, James, Sp., E. 0. Ulrich, Journ. Cincinnati Soc. Nat. Hist., 



Vol. v., p. IGO, Plate VL, figs. 8,-8c, 1882. 



Locality and Formation. — Not rare in tho Trenton Formation, near 

 Montreal City. 



Collectors.— T, C. Weston, T. Curry. 



III. On Two Species of Tetradlum from the Trenton and Hudson liiver 



Formations, 



Tetradium Peaciiii, Nich. and Eth., jun., Yar. Canadense. (N. Var.) 



Plate VI., figs. 1,-li 



In tho Trenton formation there occur very abundantly certain small, 

 rounded masses, tho organic nature of which had for a long tirao been 

 doubted. A microscopic examination of those masses has proved them to 

 bo a variety of a species found in pebbles of Upper Silurian Ago from tho 

 Devonian (Old Eed) conglomerate of Ilabbies Howe, in tho Pentland Ilills, 

 Scotland, named by Dr. Nicholson and Mr. E. Etheiidgo, Jan., Tetradium 

 Peachii.'^ The species is thus characterized by tho authors : — 



" Corallam massive, exceedingly dense and compact, composed of exces- 

 sively minute, closely approximated corallites, about a thirty-fifth of a lino 

 in diameter. The corallites have an undulating course, and are sometimes 

 disposed in superimposed layers, or arranged concontricaly round minor 

 centres. Corallites thick-walled, irregularly circular or oval in transverse 

 section, with a few (three or four ?) short septa, which are often thickened 

 at their bases. TabuUi) numerous and complete. Corallum perforated by 

 irregular tubes (water canals ?) from a fortieth to a fiftieth of an inch in 

 diameter or less." 



Tho variety occurs in tho form of very compact, irregularly rounded, 

 Hub-globoso masses, varying greatly in sizo, tho smallest measured giving 

 12 mm. in its greatest diameter and tho largest from 5 to 6 centimetres. 



It is difi^cult to detect any trace of structare, even with a hand-lens, on 

 the surface of weathered specimens, but polished surfaces exhibit a series 

 of concentric linos, arranged symmetrically around several centres. 



Through the courtesy of Dr. Nicholson and Mr. E. Etheridge, jun , tho 

 author was furnished with a specimen of Tetradium Feachii, from tho 

 typical locality, and this has been compared with tho Canadian form. In 

 its microscopic character tho variety is essentially similar to the species, 



♦Ann. Nat, Hist., Ser. 4, Vol. XX. p. IGG; figa. (7, e,fj g^ 1S77. 



