ÖPVEKSIGT AF K. VETENSK.-AKAD. FÖRHANDLINGAR 1899, N:0 2. 45 



bourhood of Halysites as Nicholson and Etheridge have pro- 

 posed.^) There are the constant four septa of Tetradium against 

 tlie twelve septa of Halysites and the chain-like arrangement of 

 the tubes in the former is only of a superficial resemblance 

 with the lameilar combinations of the calicular tubes in the 

 iatter. In the present state of our knowledge it is impossible to 

 decide where to place this Tetradium amongst the corals, if a 

 coral at all. 



What speaks for its corallian natura are some features re- 

 minding of the Palaeozoic Rugosa. There seeras to be evidence 

 that the septa, which are norm al ly four as in several Rugosa, 

 sometimes grew to an excessive length so as to meet in the centre of 

 the tube and thereby Converting a single calicle or tube into four. 

 I think that there is evidence of this shown in the figure 4 above 

 in two instances, in a smaller tube, first in the middle of the lower 

 edge, where the fission is just completed and probably a columella 

 is formed in the centre. Also in a larger one beside it to the right, 

 vv'here the four new tubes have already acquired their own septa. This 

 mode of propagating by fission is the same as in such genera as Stau- 

 ria and Ceriaster Lindström^) without in any way indicating real 

 affinity. The long grooves which run on the outside of the tubes 

 exactly opposite the septa, also give Tetradium a point of com- 

 parison with the Rugosa, for which these rugse are one of the 

 chief characteristics. 



Hitherto Tetradium has been found exclusively in various States 

 of North America, as Tennessee, Kentucky, New-York, Cincinnati 

 etc, also in Canada. The Tetradium above described is the 

 first instance of one found on this side of the Atlantic. 



As to the geological distribution of the species of Tetra- 

 dium, there can be no doubt, that, as far as our knowledge at 

 present reaches, it is restricted to the Lower Silurian formation. 

 Nicholson and Etheridge have described^) a Tetradium Peachii 



1) Ann. Mag. N. H., 4 Ser., vol. 20, pag. 165. 



^) RiCHTHOFEN, China, vol. 4, p. 61. pl. V, fig. 2 — 5. 



^) 1. c. p. 166. 



