466 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 27,
cumference is greater ; but this is owing to the circumstance that
that bone appears to be one in which the muscular ridges on the
hinder surface are very much developed, and the whole bone unusually
thick. The Oreston bone is thus satisfactorily shown to be
much larger in every way than that of R. tichorhinus.
But this is not all ; it is not only much larger, but it differs still more remarkably in its proportions. These are also shown in the Table, in which the last two columns give the ratio, first, of the least circumference to the length of the bone, which I call the " perimetral index " — and, secondly, of the antero -posterior diameter of the shaft at the middle to its transverse diameter at the same point, which ratio I denominate the " latitudinal index." Inspection of the figures in these two columns will show at a glance how much slenderer, and how very much more compressed or flattened, is the metacarpal of R. leptorhinus as compared with that of R. tichorhinus.
I have not as yet met with the metacarpal of either R. hemitoechus or R. etruscus ; but as its dimensions, and in some measure, probably, its proportions may be pretty safely predicated from those of the corresponding metatarsal, I have subjoined a Table (II.) of the dimensions and proportions of that bone in the four known quaternary species.
From this it will be seen that in R. leptorhinus the middle metatarsal is about one-eighth shorter than the corresponding metacarpal, and in R. tichorhinus about one-twelfth. Assuming that the proportion between the two bones is the same in R. hemitoechus and R. etruscus, it follows that the mean length of the metacarpal in those species would be from 7-4 to 7-6 ; and this is doubtless not far from the truth. It is clear, therefore, that, as regards length, the metarcarpal of R. leptorhinus far exceeds that of either of the other three species ; and, in fact, it is longer than in any species, living or extinct, except R. unicornis, and perhaps R. simus, of whose skeleton, however, we have no knowledge. Again it will also be perceived that although the " perimetral index " in R. hemitoechus is very nearly the same as in R. leptorhinus, the " latitudinal index " is considerably higher, or in the proportion of, perhaps, 417 to 380. This shows how much flatter or more compressed the metacarpal of R. leptorhinus is than than that of R. hemitoechus, and, as will be seen in the Table, still more strikingly than that of R. etruscus, which, to judge from the metatarsal, must be by far the most cylindrical and at the same time the slenderest of all four.
From the above considerations, I think it impossible to avoid the conclusion that the Oreston metacarpal can only belong to R. leptorhinus.
The only other bone to which I need refer is that numbered 906. It is the distal extremity, quite perfect, of the right inner metatarsal, which measures 1-8 X 1-8 in antero-posterior and transverse dimensions. These dimensions, to judge from an entire bone in the British Museum, from Grays Thurrock (no. 23761), which measures 1-7 X 1-7, would give the Oreston metatarsal a length of 7-41.
