46 LINDSTRÖM, A SPECIES OF TETRADIUM FROM BEEREN EILAND. 



from the Uppersilurian of Scotland, but after studying their de- 

 scription as well as raicroscopical slides of speciraens sent from 

 Prof. Nicholson I cannot find the distinctive characters of Tetra- 

 dium in them. The apertures of the tubes are irregulär and the 

 septa or septal spines are also irregulär in number and appea- 

 ■ rance and do not at all resemble those of Tetradium. The 

 same can be said of Tetradium eifeliense, described by Frech 

 as a Middle Devonian species,^) which is quite as irregulär and 

 unlike a Tetradiura as the former. But then Schlüter^) has 

 corrected this statement by a new description and demonstrated, 

 that the species of Frech is a CaJamopora. There seem thus 

 to be no Tetradia above the Lower Silurian. According to the 

 American authors they occur in the Trenton and Hudson river 

 groups, that is to say, the upperraost divisions of the American 

 Lower Silurian, correspouding with the Leptaena liraestone of 

 Dalecarlia and the Trinucleus shale in Sweden or with the 

 Wesenberger, Borkholraer and Lyckholmer strata of Russia. As 

 to Tetradium apertum, Safford says that it »is abundant in 

 the middle part of the Lower Silurian series of Middle Tennes- 

 see», »but also in the upper half and near the base». Further, 

 jn another paper,^) he states that the rocks of this part are 

 equivalent to the Black river groups of New York, and the 

 Trenton and Hudson River group. According to what he has 

 stated concerning the geological distribution it seems that Tetra- 

 dium apertum, like the other species of that genus occur hoth 

 in the Trenton and the Hudson River group. Ferdinand Roe- 

 MBR*) again determines the chief superior mäss of the Lower 

 Silurian strata of Tennessee as belonging to the Trenton group. 

 It is consequently at present only possible to restrict the age of 

 the Beeren Eiland limestone with Tetradium within the limits 

 of the Trenton and Hudson groups or of the Trinucleus shale 



*) Die Cyathophylliden und Zaphrentiden des deutschen Mitteldevon, p. 32. 



^) Anthozoen des rheinisclien Mitteldevon, p. 90, Taf. XI, f. 2, 3. 



^) The Silurian Basin of Middle Tennessee in Ämeripan Journ. Science and 



Arts, 2ci Ser., vol. XII, p. 354. 

 *) Die Silurische Fauna des westlichen Tennessee, p. 1. 



