ANNIVERSARY ADDRESS OF THE PRESIDENT. 51 



Prof. R. Owen,. who has described the greater number of species 

 found in British rocks, has employed for their distinction chiefly the 

 form of the teeth, the proportion of the length of the snout to that 

 of the rest of the head, and the proportionate length of the head to 

 that of the vertebral column. The well-known difference of length 

 of the snout in the same species of extant crocodiles in immature 

 and mature individuals suggests caution in the use of such propor- 

 tionate length as specific characters for Ichthyosauri, since the 

 criteria for inferring the age are less trustworthy in Reptiles, where 

 many sutures and cartilaginous parts of the skeleton persist as such 

 throughout life, than they are in higher Yertebrata, where they dis- 

 appear with maturity, and their presence is a stamp of immaturity. 



As regards their distribution in time,Enaliosauria, as is well known, 

 range from the Lias to the Chalk inclusive, where they appear to 

 have become extinct ; unless the survival of one species of Ichthyo- 

 saurus, I. gandavensis, in early Tertiary times be confirmed by 

 fresh evidence. 



To the question of greatest moment to the stratigraphist, whether 

 particular species of Ichthyosaurus and Plesiosaurus can be safely 

 taken as stamping particular geological horizons, I must reply that 

 our present knowledge does not appear to me to justify this use. 



I have referred to the difficulty in seizing good specific characters 

 for Ichthyosauri. A similar difficulty is felt with respect to the 

 members of Conybeare's original genus Plesiosaurus. Prof. H. G. 

 Seeley, in a suggestive paper read 24th Nov. 1874, and printed in 

 the Quart. Journ. Geol. Soc. vol. xxx. p. 436, showed the existence 

 of certain well-marked differences in the form and proportions of 

 the elements of the shoulder-girdle, and, assigning to these a generic 

 value, he broke up the original genus into several subgenera, of 

 which Colymbosaurus and Ophthalmosaurus, Murcenosaurus and 

 Eretmosaurus may be cited as examples. 



From 1821, the date of Conybeare and De la Beche's paper to 

 which allusion was recently made, until 1841, the genus Plesiosaurus 

 was the only representative of the order Sauropterygia discovered 

 in British rocks. In 1841, Prof. R. Owen removed out of this 

 genus two species, Plesiosaurus grandis, Owen, and P. trochanteric, 

 Owen, and made for them the new genus Pliosaurus. The fossils 

 on which this genus was first founded were two propodial bones, 

 of which one, preserved in the British Museum, was originally in 

 the collection of the Earl of Enniskillen, then Viscount Cole ; and 

 the other, preserved in the Museum at Dorchester, was obtained by 

 J. C. Mausel, now J. C. Mansel-Pleydell, Esq., in Kimmeridge Bay. 

 To the generic characters furnished by these fossils others, sup- 

 plied by teeth and cervical vertebras, were subsequently added. 

 Still later, in a paddle from the Portland quarries, Prof. R. Owen 

 found another generic character of Pliosaurus, the presence of 

 a third bone in the second segment of the limb corresponding to 

 the cnemion or antibrachion (leg or forearm) of higher vertebrates. 

 Lastly, in his paper lately read here, Prof. R. Owen makes known 

 another generic character of Pliosaurus in the presence of a dorsad 



