GIANT TUETLE FEOM QUEENSLAND LOWER CRETACEOUS.— LONGMAN. 29 
In the eetal view, shown in PI. XII, the sux’face of the plate is 
approximately fiat, but in the centre a deep depression, scar and transverse 
fracture mark a position where the limb-bones had been crushed in during the 
process of fossilisation. Except in the region of the furrows adjoining digital 
areas, the ental side is convex in section through the short axis. The exposed 
section on the left shows a number of diploic cavities. 
The possibility of this plate being regarded as a left hyoplastron not 
suturally connected with its accompanying hypoplastron was at first considered. 
The development of plastral bones in existing Chelonidm leaves at first large 
vacuities between these portions on each side. These may be noted in the plastra 
of newly hatched turtles, and the development is beautifully illustrated in W. K. 
Parker’s monograph on the structure and develo])ment of the shoulder-girdle 
and sternum in the Vertebrata.^® But the evident length of the completed bone in 
comparison with the width presents difficulties here, and the dimensions and 
contours also put out of court the probability of a united hyo- and hypoplastron. 
The writer therefore suggests that the plate may be the centre and greater part 
of the left side of a large entoplastron. Taking this view^ into consideration the 
bone may be compared to the entoplastron of Archekm ischjjros, figured by 
AVieland.^^ The dactyloid processes do not show correspondence, but here we 
should expect diversity, and there is much variability in these even in the same 
species at the present day. Assuming that the left of the fragment approximates 
to the centre of an entoplastron, there is no sign of a nether tubercular process, 
though the incomplete state of the plate does not warrant a statement that it is 
absent. It may thus have no affinities with the peculiar T-shaped entoplastron 
characteristic of the Protostegince. Taking for granted that the centre of the 
bone is situated near to the left margin, an entoplastron about 4 feet in width is 
outlined. Following out our comparisons, Ave find tliat the breadth of our large 
C. mydas is well over 2 feet in the entoplastra) region; but the contours of the 
anterior plastral bones here give scope at the side for the fore-limbs, and this 
cannot be imagined for the outwardly curved entoplastron of Archelon. But 
Wieland has pointed out that a greater breadth and “a quite orbicular form” is 
characteristic of Cretaceous turtles, and tlius an extended entoplastron comes 
Avithin the compass of our proportions. Probably such a plastral bone occupied 
a position less anterior than its homologue in modern turtles. 
Remembering the faulty allocation of fragments by more than one 
authority in the past, the Avriter has some diffidence in thus definitely placing 
this bone, but he has taken the view that the flatness of the plate and the presence 
of such lateral and posterior dactyloid processes preclude the possibility of its 
being a nuchal. 
K. Parker, Pay Society, X868, pi. xii. 
“Wieland, Annals Carnegie Alusemn, vol. iv., No. 1, 1906, p. 11. 
