102 REPORT— 1839. 



the whole of the anterior and posterior surfaces, giving to the 

 bone the appearance of its being composed of two ribs anchy- 

 losed by their sides. Mr. Clift has given a faithful view of this 

 structure in PL XIX. of the Philosophical Transactions for 1814. 

 It is gradually lost in the ribs at the posterior part of the tho- 

 racic-abdominal cavity, which from this part preserve the form 

 of simple osseous styles. The inferior or sternal extremities of 

 the opposite ribs are not immediately united together, but the 

 long hoop is completed by a sterno-costal arc composed of 

 transverse styles, which are more slender and fewer in number 

 than in the Plesiosaurus. 



In comparing the vertebras in different parts of the spine of 

 the Ichthyosaurus, modifications of structure present themselves 

 which are somewhat analogous, though minor in degree, to those 

 already described in the Plesiosaurus. The principal difference 

 to be noticed is that the lower tubercle for the attachment of the 

 rib never wholly quits the centrum : any detached vertebral 

 centrum, therefore, that might be discovered, which had no 

 lateral tubercle or articular surface for a rib, might be safely 

 pronounced, whatever the form of its anterior and posterior ar- 

 ticular surfaces, not to have belonged to a true Ichthyosaurus, 

 provided it was not compressed laterally, as in the small termi- 

 nal ribless caudal vertebrae which supported the caudal fin in 

 the Ichthyosaurus. 



In the anterior sixteen vertebrae of the Ichthyosaurus commu- 

 nis, or for a third part of the spine extending between the cranium 

 and pelvis, the lower costal tubercle only is developed upon the 

 body, the upper tubercle or articular surface resting on the neur- 

 apopliysis, or not being distinct from the neurapophysial ar- 

 ticular surface. In the twenty succeeding vertebrae, both the 

 costal tubercles are developed on the side of the centrum below 

 the neurapophysial depression. The upper of these tubercles is 

 at first placed close to the neurapophysial pit, and thence takes 

 gradually a lower position on the side, so as to approach more 

 nearly to the inferior tubercle ; at length near the 40th vertebra, 

 at a short distance beyond the iliac bones, the two tubercles 

 blend together and form a single ridge. This ridge as the 

 caudal vertebrae recede from the trunk gradually changes its 

 obliquely elongated direction for a transverse one, or becomes a 

 rounded tubercle ; and at length disappears about the 80th 

 vertebra. It is at this part of the spine in the Ichthyosauri 

 communis and intennedius that the abrupt bend or dislocation 

 of the tail usually takes place ; and here three or four of the 

 vertebral centres are more compressed than those which im- 

 mediately precede or follow them, and their margins are raised, 

 as if by forcible compression. The caudal vertebrae are more 



