1898] STRUCTURE AND HABITS OF PLESIOSA URIA 49 



of the vertebral column of the neck in comparison with that of the 

 trunk, I cannot regard as proved either the outer form just men- 

 tioned or the easy power of motion of the neck. 



If the head had been fixed at an angle to the vertebral column, 

 the occipital condyle must have been more or less sharply turned 

 downwards, which is not the case, as will be seen from a glance at 

 the figures of the skull of Plesiosaurus rostratus and P. hawkinsi 

 given by R Owen. Moreover, the skulls always lie in the same 

 stratum as the vertebrae, and almost always in direct continuation 

 of them. The neck-vertebrae increase very gradually in all dimen- 

 sions from the head towards the trunk ; the neural spines which 

 become higher and higher backwards, as also the strong cervical 

 ribs, indicate a well-developed muscular covering which was capable 

 of moving the neck as a whole, but not its separate parts one on 

 another, such as is rendered possible in the neck of the bird by the 

 saddle-shaped articulations and the almost uniform size of the 

 vertebrae from the first to the last. So highly developed a power 

 of motion is rendered improbable by the almost flat articular ends 

 of the neck-vertebrae of most species, as also by the high neural 

 spines, and especially by the double-headed cervical ribs, which 

 overlap each other like those of the crocodiles. The Plesiosauria 

 could indeed move their neck powerfully and quickly in all direc- 

 tions, but more like an elastic rod as a whole, not in S-shaped bends 

 like the neck of a bird. This conclusion, based on the structure of 

 the neck-vertebrae, also receives important confirmation from the 

 arrangement of the more or less completely preserved skeletons. 

 The drawings of complete skeletons in the works of Conybeare, E. 

 Owen, T. C. Winkler, and Sollas show that the large majority of 

 individuals are preserved with the neck quite straight and lying in 

 the long axis of the animal, or at most only slightly curved. In the 

 latter case the curvature is met with only in the front half, while 

 the hinder portion extends in a straight line. This is also the case 

 in the skeletons in which the maximum curvature is attained, namely 

 those of Plesiosaurus macrocephalus and P. brachycephalus, E. Owen, 

 both with a remarkably large head and short neck ; but even here 

 the curvature of the front portion is very far from being a semi- 

 circle. 



It is, moreover, to be remarked that the neck was not sharply 

 separated from the body, but gradually passed into the trunk, 

 because the vertebral centra are similar in size, and the neural 

 arches correspond exactly in height and breadth with those of the 

 last neck- vertebrae. This proves also that the long bands of muscle 

 which extended along the upper part of the neck-vertebrae passed into 

 the front of the trunk without an increase in size ; and thus the 

 hinder part of the neck-region must have been as thick as the front 



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