as they possess a strong bony arch, supporting the fore-limbs, such as would permit of partial, if laborious, terrestrial progression.  The head is of enormous size, with greatly prolonged jaws, holding numerous powerful conical teeth lodged in a common groove.  The nature of the dental apparatus is such as to leave no doubt as to the rapacious and predatory habits of the Ichthyosaurs—­an inference which is further borne out by the examination of their petrified droppings, which are known to geologists as “coprolites,” and which contain numerous fragments of the bones and scales of the Ganoid fishes which inhabited the same seas.  The orbits are of huge size; and as the eyeball was protected, like that of birds, by a ring of bony plates in its outer coat, we even know that the pupils of the eyes were of correspondingly large dimensions.  As these bony plates have the function of protecting the eye from injury under sudden changes of pressure in the surrounding medium, it has been inferred, with great probability, that the Ichthyosaurs were in the habit of diving to considerable depths in the sea.  Some of the larger specimens of Ichthyosaurus which have been discovered in the Lias indicate an animal of from 20 to nearly 40 feet in length; and many species are known to have existed, whilst fragmentary remains of their skeletons are very abundant in some localities.  We may therefore safely conclude that these colossal Reptiles were amongst the most formidable of the many tyrants of the Jurassic seas.

[Illustration:  Fig. 177.—­Plesiosaurus dolichodeirus, restored.  Lias.]

The Plesiosaurus (fig. 177) is another famous Oolitic Reptile, and, like the preceding, must have lived mainly or exclusively in the sea.  It agrees with the Ichthyosaur in some important features of its organisation, especially in the fact that both pairs of limbs are converted into “flippers” or swimming-paddles, whilst the skin seems to have been equally destitute of any scaly or bony investiture.  Unlike the Ichthyosaur, however, the Plesiosaur had the paddles placed far back, the tail being extremely short, and the neck greatly lengthened out, and composed of from twenty to forty vertebrae.  The bodies of the vertebrae, also, are not deeply biconcave, but are flat, or only slightly cupped.  The head is of relatively small size, with smaller orbits than those of the Ichthyosaur, and with a snout less elongated.  The jaws, however, were armed with numerous conical teeth, inserted in distinct sockets.  As regards the habits of the Plesiosaur, Dr Conybeare arrives at the following conclusions:  “That it was aquatic is evident from the form of its paddles; that it was marine is almost equally so from the remains with which it is universally associated; that it may have occasionally visited the shore, the resemblance of its extremities to those of the Turtles may lead us to conjecture:  its movements, however, must have been very awkward on land; and its long