Plesiosaurus

Plesiosaurus (Gr. plesios, 'near to,' sauros, 'a lizard'), the type or leading genus of a family (Plesiosauridae) of fossil sea-reptiles, which are characteristic of the Mesozoic systems. The skull of Plesiosaurus is small and depressed, with a short mandibular symphysis. The teeth (sunk in distinct sockets) are long, slender, and cylindrical, and show fine longitudinal ridges on the enamel. The most striking peculiarity of the vertebrae is the great length of the neck portion, which was composed of from 24 to 41 vertebrae—the anterior ones being generally very small. The cervical vertebrae consist of a centrum, neural arch, and two ribs, which are firmly articulated to the centra of the vertebrae, the terminal faces of which are more or less biconcave. In the dorsal vertebrae the ribs are articulated to diapophyses from the neural arch; and in the tail they gradually descend again to the sides of the centrum. The tail is much shorter than in Ichthyosaurus (q.v.). In the abdominal region the extremities of each pair of ribs are connected below by the development of the hæmal spine. The scapulæ are of moderate size and widely separated. The two pairs of limbs correspond closely in structure. The humerus and femur are comparatively short and distally much expanded; the radius and ulna, tibia and fibula are short and flat; the 'hand' and 'foot' are long, the phalangeals being increased in number beyond the normal complement. The limbs were covered with integument so as to form simple undivided paddles, as in the turtle. Plesiosaurus was undoubtedly aquatic, and probably haunted the shallow seas and estuaries of Mesozoic times. Its remains have been met with frequently in a fine state of preservation, some almost perfect skeletons having been obtained from the Lias of England. Several genera of Plesiosauridæ have been determined. One of these, Cimoliosaurus, met with too often cause the abandonment of all exercise, by which the morbid condition is aggravated. The state of plethora thus gradually induced may be extreme without any functions materially failing, and yet the subject is on the verge of some dangerous malady, such as apoplexy, or structural disease of the heart or great vessels, or of the lungs, kidneys, or liver.