Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/216

rivalled in proportions the largest living and extinct land tortoises of the Mascarene and Galapagos Islands. Several pieces of plates show thicknesses varying from 2 to 20 millimetres, but present no further characters of importance, all being extremely fragmentary. They establish, however, much variability in the dimensions of their owners, which is confirmed by a study of the following bones. It may be observed that the denser outer dermal layer of several fragments belonging to the small Chelonians is marked by numerous white specks, such as are seen on the epidermis of the Lutremys europæa, with which, it will be seen, the humerus (Plate VI. fig. 6) and femur (Plate VI. fig. 5) agree in all particulars.

 

Vertebral Column.

A cervical vertebra from Mnaidra Gap and a caudal vertebra from Benghisa Gap belong to gigantic land-tortoises. Both are referred to in my previous communication^[1], which was drawn up in Malta during the progress of the explorations, when I had not the means of making comparisons.

The cervical vertebra is much injured: the anterior portion is lost, leaving the posterior condyle and posterior zygapophyses, with a portion of the neural arch; the last, however, is distorted and crushed. Enough remains, nevertheless, to show that it is a fifth cervical. As compared with the same bone in an individual of the large Galapagos form described by Dr. Günther^[2] under the name of Testudo elephantopus of Harlan, the above represents not only a larger but also a more robust tortoise; and as the latter character will be seen to prevail in all the large Chelonian remains in our united collections, I propose to distinguish this (the largest) species of tortoise from the others by the name of Testudo robusta. As far as the injured condition of the fossil will allow, the following comparisons have been made between it and the typical specimen of T. elephantopus, as given by Günther. In both, the neural crest divides and proceeds along the dorsal aspects of the posterior zygapophyses, thereby forming a shallow triangular space between them. The following measurements are procurable:—

	T. elephantopus.	T. robusta.
	millim.	millim.
Breadth of condyle ................................................................................................................................................................................................................................................................................................................................................................................................	33	37
Thickness of condyle ................................................................................................................................................................................................................................................................................................................................................................................................	20	26
Greatest breadth of zygapophysis ................................................................................................................................................................................................................................................................................................................................................................................................	10	18
Least breadth of centrum ................................................................................................................................................................................................................................................................................................................................................................................................	20	22

1. Quart. Journ. Geol. Soc. vol. xxii. p. 595.
2. Philosophical Transactions, vol. clxv. 1875, p. 251.