102 
PLESIOSAURS ; 
with a shell, and the lower aspect of the same protected by a plastron, the 
plesiosaurs were entirely naked, the plastron being represented by a numerous 
series of abdominal ribs, each composed of three pieces, forming a forwardly- 
directed angle. The skull differs from that of the crocodiles in having but one 
(lower) temporal arch; and the jaws are furnished with a number of pointed and 
grooved teeth, implanted in distinct sockets; one of such teeth being figured 
on p. 5. The neck was generally much elongated, and its vertebrae differ from 
those of crocodiles in that their ribs which may have either single or double 
restored skeleton of a plesiosaur (greatly reduced). 
heads, are articulated only to the body of each vertebra (as shown in the accom¬ 
panying figure); those of crocodiles always having two heads, of which the 
lower is articulated to the body, and the upper to the arch of the vertebra. 
Throughout the backbone the bodies of the vertebrae have either nearly flat or 
slightly cupped articular surfaces; and in the region of the back each pair of ribs 
is articulated to a process arising from the arch of each vertebra, instead of 
from a facet placed at the junction of two vertebrae, as in the tortoises. Although 
there are other interesting features in these reptiles, those mentioned distinguish 
them from crocodiles and dinosaurs on 
the one hand, and tortoises and turtles 
on the other. 
With regard to the various groups 
into which the order is divided, it may 
be mentioned that in the typical forms, 
constituting the family Plesiosaur idee, 
the limbs, as shown in the figure on p. 102, 
are converted into flattened paddles, with 
a shortening of the bones of the upper 
segments, and an increase in the number 
of bones corresponding with those of the 
toes of ordinary reptiles. In the true 
plesiosaurs ( Plesiosaurus ) of the Lias, the 
ribs of the neck were articulated to the vertebrae by two heads; whereas in the 
later cimoliosaurs ( Gimoliosaurus ) of the Oolites and Chalk, such ribs, as shown in 
the figure of a neck-vertebra, were single-headed. Some of these creatures were 
of huge size, attaining a length of between 30 and 40 feet; certain of the species 
UPPER surface of the skull of the simosaur, 
OR TRIASSIC PLESIOSAUR. 
n, nostrils ; or, eye-sockets ; st, temporal fossae 
(i nat. size).—From Gaudry. 
