give the centra some resemblance to those of Ichthyosaurian vertebrae, from which, however, they may be at once distinguished by the absence of costal tubercles from the outer surface, and by the circumferential swelling of the articular face. But these vertebrae are sharply differentiated from those of Ichthyosaurus by their anchylosed neurapophyses bearing long transverse processes. These are laterally compressed blades directed upwards and outwards from the neural arch. Their outer end is slightly swollen, and cut obliquely, which makes the upper border of the process longer than the lower one, and gives the terminal rib-bearing surface an almost directly outward aspect.
The anterior articular processes spring from the crown of the arch, immediately above the neural canal. Their lower ends meet, and their upper ones are incurved, so that together the two prezygapophyses form about two-thirds of a circle, each half of which looks inwards and embraces the corresponding posterior articular process, the shape of which, and the aspect of its articulating surface, are the counterparts of those of the anterior one. The posterior articular processes spring from the back of the arch just below the root of the neural spine, in a rather higher level than that of the anterior. The neural spines are long and straight.
The ribs have a simple flattened head, separated by a slight constriction from the angle 3-1/2 inches distant, beyond which they sweep outwards and downwards in an even curve. Their upper, convex border is thick, the lower one is thin, and a shallow groove parallel to the upper one channels the surface of the rib.
The digital phalanx had the usual cylindrical figure, with constricted middle of a Plesiosaurian digit-bone. It is 1.5 long, . broad at ends, .5 broad at middle.
Plesiosaurus Manselii (Pl. XLI. figs. 1-6).
In November 1868 Mr. Mansel kindly sent me for examination a large series of vertebrae, evidently comprising the greater part of the spinal column of a Plesiosaurus, with pieces of the two humeri, and several incomplete flat bones, which some years before had been discovered in the cliffs east of Clavell's Tower, in Kimmeridge Bay.
Mr. Mansel informed me that he had himself supervised their exhumation, and that the bones were associated in such a manner as to leave no doubt in his mind that they had all belonged to one individual ; and he also mentioned that, under the impression that they were Pliosaurian remains, he had enclosed the four great paddle-bones with carpals and many phalanges in a case of Pliosaurian fossils which he had presented, to the British Museum. It was Mr. Mansel's first impression that all the four limb-bones had been enclosed in this case ; but the parts actually sent were the nearly complete left femur, the proximal half of the right one, and the distal half of the left humerus. On recently taking to the Museum the proximal halves of the humeri which Mr. Mansel sent me in November 1868 (with the spinal column) one was found to fit exactly the distal fragment of the humerus already in the Museum ; and the national collection now contains the nearly perfect femur and left humerus
