Mr. T. Stock on the Genus Tristychius. 177 
that the earth was covered with oaks, magnolias, willows, &c. 
before the Tertiary began. Gradually we are gathering the 
details of this wonderful history, and ultimately we shall be 
able to report the facts with a good degree of fulness ; but the 
causes which inspired the revolutions that have taken place 
in plant life, and the processes by which these great changes 
have been effected, seem to be as inscrutable as ever. 
XXIV.—On the Structure and Affinities of the Genus Tris- 
tychius, Agass. By THomas sSrock, Natural-History 
Department, Museum of Science and Art, Hdinburgh *. 
[Plate VII. | 
Tristychius fimbriatus, Stock. (Pl. VIL. figs. 1 and 1 a.) 
This spine I owe to the kindness of those promising young 
naturalists Messrs. Kinnear and Anderson, who discovered 
it in the Carboniferous Limestone series at Gilmerton, near 
Edinburgh. It appears to be hitherto undeseribed. 
Description.—It is distinctly sigmoidal in shape; but the 
curvature presents an exaggerated appearance through trac- 
ture. ‘lhe restoration (Pl. VII. fig. 1 a) gives a more correct 
idea of its original shape. ‘Ihe direction ot the curve in the 
distal region is peculiar, and, indeed, almost without a parallel 
in Selachian acanthoid remains. Nevertheless I attach very 
slight importance to it as a systemratic character. It is pos- 
sibly due to disease, of which there is some evidence in certain 
pustular appearances seen near the pointed extremity of the 
spine (Pl. VII. fig. 1). It is 12 inch in length, and 14 line in 
greatest width. Its surface is smooth; but a shallow and 
wide groove occupies a nearly central position along the 
middle third of the spine. It is difficult to say how tar such 
grooves, which occur rather frequently in the spines of various 
genera and species, are normal, or whether they arise trom the 
falling-in of the walls of the spine as the soft internal part 
decays. In the case of Plewracanthus, in which this middle 
groove is often visible, the appearance is due to decay in all 
the cases that have come under my notice. But in the example 
under consideration | am inclined to think that the appear- 
ance is normal, as the walls are apparently thick and the 
* Communicated by the Author, having been read to the Edinburgh 
Geological Society, March 15, 1883, 
Ann. & Mag. N. Hist. Ser. 5. Vol. xii. 13 
