1014 MR. D. M. S. WATSON ON 
The fact that in early times tortoises with mesoplastra were pro- 
portionately commoner and more widely spread than to-day, and 
that Triassic forms may have two pairs of mesoplastra, suggests 
that the number in still earlier forms was even greater, and possibly 
- derived from a condition in “ Archichelone” where there was a 
pair of plastrals to each segment of the middle dorsal region. 
The fact that adaptation to aquatic life almost always implies 
degeneration of the shell in Chelonia, shows conclusively that 
“« Archichelone”’ must have been a land form. 
Limb-girdles.—The most striking feature of the limb-girdles of 
Chelonia is that they lie within the shell and ribs. It is plain 
that this condition was gradually acquired after the development 
of the shell by the migration of the pectoral girdle backwards, and 
that further growth of the front and back of the shell which we 
have already found necessary to account for the dissociation of 
the nuchal and pygal elements from the vertebrae to which they 
probably originally belonged. The whole arrangement is designed 
to allow of the retraction of the limbs for protection. 
~ In “ Archichelone ” the pectoral girdle was undoubtedly in the 
usual position overlying the first dorsal ribs, and must have been 
narrow relatively to the bulk of the trunk to allow of its passage 
back within the ribs. 
The pelvic girdle must also have been very narrow. 
This shows that the neck and tail of ‘‘ Archichelone” must have 
been sharply marked off from the trunk by their smaller diameter. 
It is impossible to discover the structure of the limb-girdles of 
primitive Chelonia from those we know; the pectoral girdle 
of a Triassic type recently described by Jackel shows that the 
huge acromion, which is one of the most striking peculiarities of 
the ordinary form, has arisen in comparatively recent times. There 
can be no reasonable doubt but that the girdles of “* Archichelone ” 
were of what Prof. Williston calls the “old fashioned” type, 
i.e., that there were two coracoidal elements and the pelvis was 
“plate-like.” - 
Limbs.—From a study of the mode of walking of land Chelonia 
it seems to be possible to arrive at some conclusions as to 
the type of limbs present in ‘ Archichelone.” In tortoises 
the fore arm and leg are carried nearly at right angles to 
the upper arm and thigh and stand almost vertical, sometimes 
even standing over at the elbow. The humerus and femur are 
carried nearly horizontally and very seldom depressed below the 
level. The animal sways from side to side, so as to bring its 
centre of gravity within the area of support afforded by the three 
legs it stands on whilst the fourth is advanced. 
Some of these features, the standing over at the elbow for 
example, are obviously due to the development of the shell; but, 
even if allowance be made for this, there remains sufficient to 
suggest that the Chelonia were derived from a group with the 
type of humerus found in Lryops and Dimetrodon, which had a 
