14 DR. R. W. GIBBES ON THE 



the anterior face is irregularly concave, with a depression in the centre, while the pos- 

 terior is slightly convex ; in the smaller vertebrae (fig. 4) which have no processes, the 

 concavo-convex character is more marked. Several of the larger vertebrae resemble 

 the description given by Dr. Mantell* of Cctiosaurus; but instead of becoming bi-con- 

 cave as they approach the caudal extremity, the posterior surface becomes more 

 convex. In this respect they resemble the Monitor. All the caudal vertebrae of the 

 Monitor have the anterior surface concave, and the posterior convex. PI. II., fig. 8, 

 represents the caudal vertebrae in position. 



The centrum or body of the six larger vertebrae, which differ very little in size, is 

 somewhat contracted in the middle ; and on the sides of the processes is a little de- 

 pressed : the transverse processes are pyramidal, the base extending the whole length 

 of the body, next to which, in the centre, is a large oval perforation. This I find in 

 some of the vertebrae in Prof Warren's collection. He has no small caudal vertebrae. 

 On the abdominal surface of the body is a longitudinal depression, which might 

 almost be called a groove, in the centre of which is a large foramen, in all the 

 vertebrae. It is remarked by Professor Owen,f that the bodies of the vertebrae in 

 most species of Plesiosaurus, are traversed vertically by two vascular canals, which 

 lead from the medullary or spinal canal to the inferior surface of the centrum, where 

 they terminate each by an orifice, and sometimes by two orifices, on each side of the 

 middle line. These orifices, however, are not a constant character of the genus 

 Plesiosaurus, neither are they peculiar to this genus, being present in the vertebrae 

 of the Cetacea as well as those of the Sauria. The medullary cavity is formed by 

 two lateral processes extending upward and forward, connected by an arched, bony 

 band. All the apophyses seem anchylosed, for I can find no trace of a suture. In 

 the larger specimens are tubercles for the attachment of chevron bones. 



In making a distinction between my two specimens, in addition to the more elon- 

 gated and tapering form of the jaw, I urge the difference in form of the tusks, the 

 oblique position of the fangs, and the freedom from the projecting inner plate of the alve- 

 olus ; and I still thinlc the character of hoUowness is not entirely to be set aside. Prof. 

 Wyman is of opinion that the thinness of the dentine and the hoUowness are indicative 

 of the immaturity of the animal ; while Prof. Agassiz inclines to the belief that it is not 

 a young animal, as the enamel is perfectly developed and smooth — not wrinkled as 

 in young animals. Apart from this character he considers it a different species from 

 Z. cetoides. V/ith these impressions, I propose for it the specific name of serratus. 



The question now arises whether Dr. Harlan's original name of Basilosaurus should 

 or should not be restored to the remarkable remains now under consideration. That 

 it is objectionable in a critically zoological point of view, we grant ; but it is not 

 more so than many other names that are admitted in the systems, among which we 



. * Medals of Creation, p. 727. t Report on British Reptiles. 



