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. Ichthyostwrus commu 



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- 



rus 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. 



