SWIMMING ANIMALS. 55 



into paddles had not been carried to the same degree 

 as in the Ichthyosaurs. The evidence for the derivation 

 of the Plesiosaurs from terrestrial reptiles is even fuller 

 than in the case of the group last mentioned. 



Before taking leave of the reptiles we have to allude 

 to another totally different assemblage of extinct aquatic 

 forms, which were much more closely allied to the 

 existing Lizards, and many of which were of gigantic 

 dimensions. These creatures are generally known as 

 the Mosasaurs, and were first brought to notice dur- 

 ing the last century, when a huge skull was obtained 

 from the upper Cretaceous beds of Maastricht, on the 

 Meuse ; the Latin name of the group being taken from 

 that river. 



These Mosasaurs, of which a large number of kinds 

 are now known, differ from the Ichthyosaurs and 

 Plesiosaurs in that the joints of their backbone, in- 

 stead of having both front and back surfaces either 

 deeply cupped or nearly flat (Fig. 24) had cup-and- 

 ball articulations, the cup occupying the front surface. 

 This type of structure is common to existing Crocodiles 

 and Lizards ; but whereas in the former the ribs 

 articulate with the joints of the backbone by means of 

 long transverse processes jutting out from them, in the 

 latter the ribs articulate directly with the aforesaid 

 joints. Now the Mosasaurs have the latter mode of 

 articulation, and, since they agree with the modern 

 Lizards in the structure of their skulls, as well as in 

 many other points of their bony anatomy, there can be 

 no hesitation in regarding them as a group descended 



