46 REPORT — 1839. 



Ichthyosaurus are the types ; and I next proceed to consider the 

 general characters of these two primary divisions of the order. 



These characters are mainly derived from modifications of 

 the vertebral column, as well with regard to the form and confi- 

 guration of the individual bones, as to the relative propor- 

 tion of the different groups of vertebrae called cervical, dorsal, 

 caudal, &c. The vertebrae also frequently afford the best 

 characters for the distinction of species, as well as of genera ; 

 and as they are the parts of the skeleton most commonly 

 discovered in the strata characterized by the Enaliosam-ian 

 remains, they have received especial attention in the present 

 Report. 



To save much repetition, otherwise unavoidable in the subse- 

 quent pages, and to facilitate the comprehension of many of 

 the descriptive details, it will be advantageous to premise some 

 observations on vertebrae in general, before entering upon the 

 modifications of these bones which characterize the Plesiosauri 

 and Ichthyosauri respectively. 



At the commencement of my examination of the fossil re- 

 mains of the Enaliosaurians, I endeavoured to apply to the 

 parts of the vertebrae, which in these animals are frequently 

 complicated, and with the elements more or less dislocated, the 

 views and nomenclature of M. Geoffroy St. Hilaire, whose ana- 

 lysis of a vertebra in the abstract has been generally adopted in 

 this country. I was soon compelled, however, to relinquish 

 the advantage which the vertebral theory of that philosophical 

 anatomist seemed to promise ; finding that it did not agree with 

 my observations either on the cartilaginous or osseous centres 

 as they appear in the development of a vertebra in the embryo ; 

 or on the fully- developed elements as they are exhibited in dif- 

 ferent classes of the vertebrate series, more especially in certain 

 parts of the vertebral column of the Plesiosaiirus. 



I need hardly observe that a vertebra may be traced through 

 its various degrees of complication, either during the progressive 

 stages of its development, or by taking permanently-formed 

 vertebrae of different grades of complexity in different animals ; 

 or, in many instances, by comparing the vertebrae in different 

 parts of the spine in the same animal. 



The terminal vertebrae of the tail in most species exhibit the 

 simplest condition of these bones. The most complicated ver- 

 tebrae which 1 have yet met with, are those at the lower part 

 of the neck of certain birds, as the Pelican ; or at the beginning 

 of the tail of a Python, or other large serpent. 



The parts or processes of such a vertebra maybe divided into 

 autogenous^ or those which are independently developed in 



