ICHTHYOPTERYGIA. 



369 



sacrum, and no sternal ribs or sternum, but clavicles were pre- 

 sent, as well as an interclavicle (episternum) ; and false ribs 

 were developed in the walls of the abdomen. The skull had 

 enormous orbits separated by a septum, and an elongated 

 snout. The eyeball was protected by a ring of bony plates in 

 the sclerotic. The teeth were not lodged in distinct sockets, 

 but in a common alveolar groove. The fore and hind limbs 

 were converted into swimming-paddles, the ordinary number 

 of digits (five) remaining recognisable, but the phalanges being 

 greatly increased in number, and marginal ossicles being added 

 as well. A vertical caudal fin was in all probability present. 



Fig. 319. Ichthyosaurus communis. Lias. 



The order Ichthyopterygia includes only the gigantic and 

 fish-like Ichthyosauri (fig. 319), all exclusively Mesozoic, and 

 abounding in the Lias, Oolites, and Chalk, but especially char- 



Fig. 320. Two vertebrae of Eosaiirus Acadiensis (Marsh). Coal-measures of 

 Nova Scotia. (After Dawson.) 



acteristic of the Lias. There is no doubt whatever but that the 

 Ichthyosauri were essentially marine animals, and they have 

 been often included with the next order (Sauropterygia) in a 

 common group, under the name of Enaliosauria or Sea-lizards. 



2 A 



