Geological Society, 65 



Richard Owen, Esq., F.G.S., Hunterian Professor in the College of 

 Surgeons, London. 



The author premises his description of the Plesiosaurus macroce- 

 phalus, by pointing out the characters of a species of Plesiosaurus, 

 which he regards as distinct from the Plesiosaurus dolichodeirus of 

 Mr. Conybeare ; and which, from the completeness of its skeleton in 

 the British Museum and other collections, he selects for a more im- 

 mediate comparison with the Plesiosaurus macrocephalus. 



He proposes to call the species thus selected, as a term of com- 

 parison, Plesiosaurus Hawkinsii, in honour of the gentleman to whose 

 remarkable skill and indefatigable labour, the beautiful and perfect 

 skeletons of it are exclusively due. The chief points in which the 

 Plesiosaurus Hawkinsii differs from the PI. dolichodeirus are, — that 

 the neck is a little longer than the trunk, instead of being fully equal 

 to the body and tail united; — that it contains twenty-nine cervical 

 vertebrae, bearing hatched-shaped ribs, instead of thirty-five; and that 

 the length of the head is equal to one-tenth part of the total length 

 of the skeleton, instead of one-thirteenth part as in the dolichodeirus. 

 The PL Hawkinsii differs also in the relative shortness and form of 

 the ulna and fibula, and in some other minor points. 



Having defined the species selected to illustrate the specific pecu- 

 liarities of the PI. macrocephalus, Mr. Owen next offers some new 

 views respecting the elementary composition of a vertebra in the abs- 

 tract, suggested principally by a study of the vertebral column in 

 the Plesiosauri ; for having observed that the vertebral ribs, or the 

 elements termed by Geoffroy St. Hilaire paraaux, or para-vertebral 

 elements, are not bent down in the caudal region to form the protect- 

 ing lamina? of the vascular trunks beneath the tail, but are continued 

 as shorter rib -like processes through a great part of the tail, co* 

 existing with the inferior lamina? (also called paraaux by Geoffroy), 

 he proposes to call these latter or inferior elements (which remain 

 united in the Plesiosauri) ' hcemapophyses' , in allusion to their physio- 

 logical relations with the great blood vessels. The superior lamina? 

 he denominates on the same principle ' neur apophyses' , from their 

 being developed to protect the great nervous trunk. The author fur- 

 ther observes that the parts or processes of a complicated vertebra 

 are of two distinct kinds ; some being developed independently in 

 separate cartilages, while others are mere projections from these in- 

 dependent constituents. 



As examples of the first, or autogenous elements, Mr. Owen in- 

 stances the centrum, or body of the vertebra? ; the neur apophyses and 

 superior spine ; the hcemapophyses and inferior spine ; and the ribs, or 

 Ann. Nat. Hist. Vol.2. No. 7. Sept. 1838. f 



