ON A NEW SPECIES OF PLESIOSAURUS 527 



In No. 3 the dorso-sacral vertebras are hidden in the same way as 

 in No. I. 



In No. 4 the head and anterior cervical vertebrae are removed, and 

 the dorsal region is dislocated, the hinder part of the vertebral column 

 ■overlapping the anterior. 



No. 5 alone exhibits the posterior part of the cervical and the 

 whole dorsal region undisturbed. Either the sixteenth or the seven- 

 teenth vertebra in the series, counting from the first (broken) one, is 

 here certainly the first dorsal — I believe the seventeenth. 



The forty-second vertebra is certainly caudal ; hence as there are 

 two sacrals, 42 — (i7-|-2) = 23, which is the number of dorsals in 

 P. Etheridgii, to which this specimen has in other respects a close 

 resemblance. 



Under these circumstances I can only suppose that Prof Owen 

 has some other evidence than that mentioned in his ' Report ' for the 

 following statement : — 



" From the 32nd to the 56th vertebra inclusive, the costal articular 

 surface is wholly impressed on the neurapophysis." — (Report, p. 57.) 



Now, as Prof. Owen states in his memoir on P. macrocephalus 

 (p. 527), that in the sacral vertebrae of P. Hazvkinsii "a small part 

 ■of the costal articular surface is contributed by the centrum," it 

 necessarily follows that these twenty-five vertebrae (32nd to 56th in- 

 clusive) are neither sacral nor cervical, but dorsal. It is true that at 

 page 66 of the ' Report ' Prof Owen affirms that there are only 

 twenty-three dorsal vertebrae ; but I cannot venture to set this cur- 

 sory contradiction against a definite anatomical statement like the 

 foregoing. 



I have been most desirous to arrive at a clear understanding of 

 Prof. Owen's definition of the species R Hawkinsii ; but after long 

 and careful study I can only arrive at the following alternatives : — 



Either i. The apparently contradictory statements which I have 

 quoted have been made through the use of a double definition of a 

 ■" cervical vertebra," — meaning thereby in one case a vertebra with a 

 certain kind of rib, in the other a vertebra with a certain kind of 

 costal articular facet ; — 



Or 2. Believing the number of cervico-dorsal vertebrae to be con- 

 stant in the same species, Prof Owen conceives that the special dorsal 

 modification may commence either at the 30th or at the 32nd vertebra, 

 according to individual variations. 



On this hypothesis it must be assumed that the smaller number of 

 dorsal vertebrae assigned to this species was found in that individual 

 which exhibited the larger number of cervicals, and vice versa. The 



