Geological Society. 



575 



the same, or five and a half inches in the vertehra which has only 

 three inches and three lines of breadth, five inches in that which is 

 two inches and nine lines broad, and four inches in that which has a 

 breadth of two inches. These eleven vertebrae do not constitute, 

 Mr. Owen shows, a regular sequence, but detached links of the ter- 

 mination of the spinal column. In all the existing genera of Cetacea 

 the posterior caudal vertebrae become shorter in proportion to their 

 thickness, and the terminal ones are depressed. The slender elon- 

 gated form of the corresponding vertebra? in the Cetiosaurus, is, Mr. 

 Owen shows, a striking crocodilian character ; and he adds, it is im- 

 portant to observe that not any of the series of caudal vertebra? de- 

 scribed in this paper exhibit the vertical canals or perforations of the 

 side of the centrum or base of the transverse process which so pe- 

 culiarly characterizes most of the cetacean caudal vertebra?. 



In his comparison between the vertebra? of the Cetiosaurus and 

 the Poikilopleuron, Professor Owen states that the caudal vertebrae 

 of the former resemble those of the latter and most other reptiles 

 from strata below the chalk in the articular surfaces being slightly 

 concave ; and the vertebrae of the Poikilopleuron, especially in the 

 elongated and rounded form of the body ; in its median compression, 

 and in the articulation of the haemapophyses to the inferior part of 

 the vertebral interspaces, though they are larger ; on the contrary, the 

 Cetiosaurus vertebrae differ in their proportions, in their structure, 

 as in the absence of the remarkable medullary cavity in the middle 

 part of the centrum of the Poikilopleuron ; in the shortness of the 

 neurapophyses as compared with the centrum ; and in other minor 

 points, which are fully detailed by Professor Owen. 



The author then proceeds to institute further comparisons between 

 the vertebra? of the Cetiosaurus and other reptilia : thus he shows 

 that they differ from the vertebrae of the Crocodilians in retaining 

 the cylindrical form of the body to the end of the tail, instead of 

 being compressed and four-sided ; that there is no trace of the ver- 

 tical median division which the bodies of the caudal vertebrae pre- 

 sent in Iguana?, Anolides and other Lacertians ; that they are not 

 only larger than in the Megalosaurus, but relatively longer ; that 

 they differ from the anterior caudal vertebra? of the Iguanodon, 

 which are nearly as large, in the absence of the well-marked con- 

 cavity below the transverse processes, in the form of the centrum 

 not being so quadrilateral, and especially in the transverse breadth 

 of the inferior surface being less ; and from the posterior caudal 

 vertebrae of the Iguanodon, which slightly increases in length, in 

 being less compressed and the centrum not having a triangular form ; 

 the slender terminal caudal vertebra? of the Iguanodon are also hex- 

 agonal, and not cylindrical as in the Cetiosaurus. 



As there is no known extinct saurian which can so nearly com- 

 pete in size with the Cetiosaurus as the Iguanodon, it is fortunate, 

 Prof. Owen observes, that the distinguishing characters are so well 

 marked and easily recognizable. 



Dorsal vertebra. — The only portion of a dorsal vertebra described 

 in the memoir is the extremity of a spinous process, the posterior 

 surface of which is rough and flattened, 4 inches across, at about 



