Radius.
This bone is represented by two specimens from Zebbug. The one is about a fourth part longer than the other; they agree, however, in every determinable particular; so that, admitting variability in size according to sexual and individual peculiarities, it seems probable that they belonged to the adult male and female of T. robusta.
The larger (Plate VI. fig. 1) has lost a portion of the outer aspect of the head and a fragment of the distal extremity; but fig. 2, also belonging to the left forearm, is quite entire.
The dimensions of these bones, as compared with one another and with the large Galapagos tortoises described by Günther[1], are as follows:—
T. robusta ♂.
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T. robusta ♀.
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T. ephippium ♂.
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T. vicina ♂.
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T. elephantopus ♂.
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| millim. | millim. | millim. | millim. | millim. | ||
|
156 | 110 | 149 | 122 | 121 | |
|
76 | 58 | 51 | 49 | 50 |
These measurements at once demonstrate the greater thickness in T. robusta of the shaft as compared with the length, the girth of even the smaller being greater than obtains in any of the more gigantic recent species.
Other comparisons as regards the articular surfaces furnish equally interesting results. As compared with the radius of the immature female of T. elephantopus referred to by Günther[2] (No. 1011 of the Cat. Mus. R. Coll. Surgeons), the smaller radius (fig. 2) is precisely of the same length, whilst the girth, midshaft, of the recent bone is 43 against 58 millims. of the smaller T. robusta.
The dimensions of the extremities of the Galapagos radii are not given by Günther; but the typical radius of T. ephippium now before me furnishes the following comparisons with the two radii of T. robusta:—
