1849.] OWEN ON FOSSIL REPTILES. 383 



the thoracic region, in which the parapophysis (jpi) has ascended to 

 the upper border of the side of the centrum, whilst the hypapophysis 

 (A) still continues to be developed from the lower surface of the cen- 

 trum. The peculiar and distinctive character of these vertebrae is 

 shown in the large size, and especially the great antero-posterior 

 extent of the hypapophysis. Its base occupies the whole extent of 

 the median line of the inferior surface between the prominent borders 

 of the anterior and posterior articular ends of the centrum ; and the 

 length of this large lamelliform hypapophysis seems to have been con- 

 siderable, since, in the vertebra, figs. 8, 10, in which upwards of half 

 an inch of its base is retained, there is little diminution of thickness 

 at the fractured surface. 



The degree of concavity of the two articular extremities of the cen- 

 trum corresponds with that in the Teleosauroids, to which family of 

 amphicselian Crocodilia these vertebrae are referable. They indicate, 

 however, a particular genus in that family, of which, from their 

 stratum, it would seem to be the latest representative ; and I propose 

 the name Hyposaurns for ihisyenus, in reference to the. characteristic 

 process — the hypapophysis, and suggest that the species, when its 

 characters are more fully worked out, should be called after the di- 

 stinguished and amiable geologist to whom we are indebted for our 

 knowledge of the existence of such a Teleosauroid in the cretaceous sera. 



DESCRIPTION OF THE PLATES. 

 PLATE X. 



Fig. 1. Side view of the third or fourth cervical vertebra of the Crocodilus basi- 



fissus. 

 Fig. 2. Under view of the same vertebra. 



Fig. 3. Side view of the corresponding vertehra of the Crocodilus basitruncatus. 

 Fig. 4. Under view of the same vertebra. 



In all these figures, c is the centrum, hy the hypapophysis, jj the para- 

 pophysis, d the diapopliysis, n the fractured base of the neural arch. 

 Fig. 5. Back view of the basioccipital bone of the Mosasaurus Maximiliani : hy, its 



hypapophyses. 

 Fig. 6. The same view of the basioccipital of an Iguana. 

 Fig. 7. The same view of the basioccipital of a young Alligator. 



PLATE XI. 



Fig. 1. Side view of a cervical or anterior abdominal vertebra of the Macrosaurus 

 lavis. 



Fig. 2. Under view of the same vertebra. 



Fig. 3. Anterior concave articular surface of the same vertebra. 



Fig. 4. Posterior convex articular surface of the same vertebra. 



Fig. 5. A smaller vertebra of the same species. 



Fig. 6. Posterior convex articular surface of the same vertebra. 



Fig. 7. Side view of the centrum of an anterior dorsal vertebra of the Hyposaurus 

 Rogersii. 



Fig. 8. Side view of a succeeding vertebra of the same species. 



Fig. 9. Under view of the vertebra fig. 7. 



Fig. 10. Posterior subconcave ai'ticular surface (partially mutilated) of the ver- 

 tebra fig. 8. 

 In all these figures d is the diapophysis, p the parapophysis, and hy the 



hypapophysis. 

 In ])oth Plates the figures arc of the natural size. 



