Prof. Owen on the genus Dicynodon. 83 



may here state, that the vertebrse of the Dicynodon present the sub-biconcave 

 structure common to most of the older extinct Saurians, which structure, in com- 

 parison with the ball and socket vertebrae of the modern species, indicates a more 

 aquatic and perhaps marine theatre of life for the Amphibia that swarmed in such 

 plenitude of development and diversity of forms during the ancient secondary 

 periods of the geological history of this planet,. 



DESCRIPTION OF THE PLATES. 



Plate III. Dicynodon lacerticeps. 



Fig. 1. Side view of the skull. 

 2. Upper view of the skull. 



Plate IV. 



Fig. 1. Front view of the skull. 



2. Back view of the skull (occiput). 



3. Upper view of the fore-part of the lower jaw. 



4. Under view of the skull. 



5. Section of lower jaw one-third from the condyle. 



6. Section of the same jaw one-third from the symphysis. 



Plate V. Dicynodon testudiceps. 



Fig. 1. Under view of anterior two-thirds of the skull, showing the bony palate. 



2. Upper view of the same portion of skull. 



3. Longitudinal section of the root or implanted part of the tusk. 



4. Transverse section of the base of the tusk. 



Plate VL 



Fig. 1. Side view of anterior two-thirds of the skull of Dicynodon testudiceps. 



2. Side view of anterior half of the skull of Dicynodon strigiceps. 



3. Under view of the same portion of skull. 



4. Portion of maxillary bone, with the root of the tusk and basal pulp-cavity exposed, 



of a young Dicynodon. 



5. Side view of the skull of an Acrodont Lizard, indicating a nondescript genus {Rhyn- 



chocephalus) by the division of the intermaxillary into two bones, as in the extinct 

 Rhynchosaurus. 



6. Under view of the same skull. 



7. Upper view of the same skull. 



