FROM THE LOWER LIAS OF CHARMOUTH. 



469 



of another* ; and sometimes, finally, they are not comparable, as 

 when in one place the length of the skull is given (as it too frequently 

 is) as the length of the lower jaw, while in another it is taken as 

 the length between the end of the snout and the basioccipital. 



On reference to the Table it will be seen that P. Conybeari has the 

 same number of cervical vertebrae as P. homalospondylus, but one 

 less dorsal : its head, however, is much larger than that of the 

 latter species ; and hence there is a great difference in their re- 

 spective cephalic indexes. In proportion of head to neck it agrees 

 exactly with P. Etheridgii, but differs widely in the number of cer- 

 vical vertebrae. It has also two dorsal vertebrae fewer ; and its dorsal 

 and cervico-dorsal cephalic indexes are distinctly different. Its 

 cervico-dorsal index approaches nearest to that of P. HawJcinsii, in 

 which species this index attains its maximum value. 



Plesiosaurus. 



Cephalic indexes. 



Number of vertebras. 



Icerrico- 



1 



Humero- 



Cervico- 

 cephalic. 



Dorso- 

 cephalic 



Cervico- 

 dorsal 

 cephalic. 



Cerv. 



Dors. 



Sac. 



Cau. 



dorsal 

 index. 



radial 

 index. 





87-6 



58-3 



339 











72-2 







63-9 



511 



28-4 



24 



24 



2 



34 



92-3 



37-4 





600 



55-5 



28-85 



25 



23 



2 



34 



92-6 



45-8 





55-5 



416 



24-0 



27 



30 



2 



32 



75-0 



28-5 



megacephalus . . . 



5.33 



493 



26-5 



30 



26 



2 



34 



92-6 



33-5 



macrocephalus ... 



51-2 



65-6 



28-8 



29 



20 



2 





128-0 



310 



brachycephalus . . . 



35-3 



403 



18-8 



31 



24 



2 



21 



114-0 



33-3 





33-8 



29-7 



15-8 



33 



25- 





32 



87-5 



34-5 





30-0 



469 



18-3 



31 



23 



2 





156-2 



34-7 



Etheridgii 



24-3 



26-0 



125 



30 



23 



2 



34 



106-0 







241 



37-4 



14-6 



38 



21 



2 



5+ 



1530 



37-0 



dolichodeirus 



177 



23-6 



10-1 



41 



21 



2 



30 -j- 



133-3 



42-9 



macropterus 



12-8 



17-0 



73 



39 



24 



1? 



28 



1320 



50-0 



homalospondylus 



10-6 



11-1 



5-4 



38 



22 



o 





1050 



46-0 



There is a fine specimen of Plesiosaurus in the British Museum, 

 with the MS. name P. laticeps, Owen, which bears a close resem- 

 blance to P. Conybeari. Its pectoral and pelvic girdles are well ex- 

 posed on their ventral surface, and are strikingly similar in general 

 form and arrangement to those parts in P. Conybeari; the dimensions 

 of their component bones also show a general agreement, as will be 

 seen from the following table. In each the humerus is longer than 

 the femur ; but these bones are each an inch shorter in P. laticeps 

 than in P. Conybeari. 



* Thus, in his Monograph on Liassic Reptiles, Professor Owen says of 

 P. rostratus that the skull is f the length of the neck. ISTow the skull is 1 foot 11 

 inches long ; and thus the length of the neck should be 2 feet 6 inches 8 lines. 

 But he also says that the length of the neck is rather less than ^ the length of the 

 spinal column ; as the latter is 9 feet 9 inches long, the neck should be "rather less " 

 than 3 feet 3 inches long. Whereabouts between these two quantities is the 

 exact length ? And would it not seem to have been easier to directly state it? 

 Unfortunately the instance here given does not stand alone. 



Q. J. G. S. Is T o. 147. 2 1 



