A. Smith- Woodward—On Rhinobatus Bugesiacus. 393 
sizes of the blocks and the various directions of their fall. The 
surface of the stone-rivers might thus be continuous with the slopes 
of the surrounding country; there would not necessarily, although 
there might, be bounding cliffs’ In the “blocky structure” of 
mountain-summits, a limit in depth must soon be reached, beyond 
which wind and frost and rain can have but little effect in weather- 
ing and removing the softer rock. But, in the stone-rivers of the 
Falklands, the process may be carried very much further: as long 
as streams are able to find their way among the blocks and can 
remove the sand between them. 
This theory seems to me to account satisfactorily for the features 
of the stone-rivers, so far as they are given in the published 
narratives. It accounts also for another fact which is not referred 
to in the theories mentioned above, namely, the proportion of the 
volume of the quartzite blocks to the volume of the rock that must 
originally have occupied the valleys. From the slight slope of the 
surface, and the certainly not small depth, of the stone-rivers, we 
must infer that this proportion is not inconsiderable. If the for- 
mation of the stone-rivers, then, began some time after the com- 
mencement of denudation in the islands, not only must the quartzite 
blocks resulting from previous erosion in some way have been 
removed, but the valleys must also have greatly increased in width 
in order to provide the material for the stone-rivers: a large amount 
of the softer rock must have been carried away: and, therefore, in 
part, at least, a cause like that suggested in this paper must have 
been in action. But if we suppose that the formation of the stone- 
rivers has all along taken place concurrently with the excavation of 
the valleys, we can, I think, account for the origin of the former 
without having to call in the aid of any non-existing agencies to 
explain the transport of the blocks. 
1V.—Nortse on Ra#INOBATUS BUGESIACUS—A SELACHIAN FISH FROM 
THE LITHOGRAPHIC STONE. 
By A. SmitH Woopwarp, F.G.S., F.Z.S. 
INCE 1857,’ the occurrence of a gigantic species of the Selachian 
family Rhinobatide in the Lithographic Stone of Bavaria has 
been well known, and three fine specimens are exhibited in the 
Munich Museum. No figure, however, appeared until 1887, when 
Dr. K. A. von Zittel published* the illustration he has kindly 
allowed to be reproduced here; and the only detailed description is 
that of Wagner,! written about thirty years ago. Quite recently, 
the British Museum has acquired the counterpart of one of the 
fossils in the Munich collection—a female individual in a remarkable 
1 « Why,’’ asks Dr. Coppinger, ‘‘do they [the stone-rivers} exhibit a margin so 
sharp and well-defined, yet without the elevated rounded appearance of a river- 
bank ?’’ 
2 A. Wagner, Gelehrte Anz. bay. Akad. Wiss. vol. xliv. (1857), p. 292. 
3 Handb. Paleont. vol. i. p. 103, fig. 117. 
4 Abh. k. bay. Akad. Wiss. Math.-phys. Cl. vol. ix. (1861), p. 313. 
