192 SAUEOPTEKYGIA. 



was evidently a considerable gap between the centra 

 marked 12 and 13 ; one vertebra (42496 a) having been 

 found since the series was described, which is evidently 

 one of the missing specimens from this part. The first 

 12 remaining vertebrae are somewhat distorted and have 

 lost the greater portion of their arches. In the ninth 

 the dimensions of the centrum are : — length 0,053 (2*1 

 inches), height 0,056 (2-2 inches), and width 0,062 (2-4 

 inches). In the later cervicals the centrum becomes rela- 

 tively shorter ; the lateral and haemal depressions being 

 well marked. The dimensions of No. 42496 a, which is 

 probably about the 18th of the complete series, are: — 

 length 0,048 (1-9 inches), height 0,053 (2-1 inches), width 

 0,067 (2*65 inches). Still later in the cervical series 

 (42496 c) the lateral depressions become less marked, 

 and the haemal depressions totally disappear, so that there 

 is a blunt ridge between the two foramina on this surface. 

 The dimensions of this centrum are : — length 0,058 (2-3 

 inches), height 0,081 (3-2 inches), width 0,098 (3*9 

 inches). Comparisons of some of the vertebrae are made 

 below. Presented by J. G. Mansel-Pleydell, Esq., 1870. 



41802 x. The third (?) cervical vertebra, wanting the greater portion 

 of the arch ; from the Kimeridge Clay of Ely, Cambridge- 

 shire. The dimensions of the centrum are :— length 0,029 

 (1-15 inches), height 0,030 (1-18 inches), width 0,036 (1-4 

 inches). The corresponding vertebra is missing in the pre- 

 ceding series, but the centrum of this specimen agrees 

 in size with that of the axis of the latter. The conjoint 

 atlas and axis of the type skeleton of P. megadirus figured 

 by Barrett in the ' Ann. Mag. Nat. Hist.' 1858, vol. ii. 

 pi. xiii. fig. 2, also agree precisely with the corresponding 

 element in No. 42496. Purchased, 1869. 



R. 1256. Casts of the centra of the 10th and 22nd cervical ver- 

 tebrae. The originals belong to the vertebral column 

 on which Plesiosaurus megadirus was founded, which was 

 obtained from the Kimeridge Clay of Haddenham, Cam- 

 bridgeshire, and is preserved in the Woodwardian Museum 

 at Cambridge. The dimensions of the 10th are : — length 

 0,041 (1-60 inches), height 0,042 (1-65 inches), width 0,052 

 (2-5 inches) ; and those of the 22nd :— length 0,054 (2-13 

 inches), height 0,063 (2-48 inches), width 0,076 (2-93 

 inches). The 10th closely resembles the early cervicals 



