PHYLUM CHORDATA 



373 



able features. The coracoids always meet in a ventral symphysis, and the 

 ventral portions (acromial processes) of the scapulae may also meet. In front 

 is, in most cases, an arch of bone, consisting of a median and two lateral 

 portions, which probably represent the episternum and the clavicles : in some 

 forms this arch is reduced or absent. Tn the pelvis the broad pubes and ischia 

 meet in the middle line : the two symphyses may 

 remain separate (Fig. 1014), or they may unite so as 

 to divide the space into two separate obturator 

 foramina. The teeth are implanted in distinct 

 sockets. 



The Sauropterygia date back to the Trias, and 

 perhaps to the Permian, extending onwards to the 

 Cretaceous. 



ICHTHYOPTERYGIA. 



The Ichthyopterygia, including Ichthyosaurus (Fig. 

 1015) and its allies, were aquatic Reptiles, some of 

 very large size (30 or 40 feet in length), with some- 

 what fish-like body, large head produced into an 





FIG. 1014. Plesiosaurus, pelvic arch. 11. ilium ; Is. ischium ; 

 Pb. pubis. (After Huxley.) 



elongated snout, no neck, and an elongated tail, 

 with a large vertical caudal fin, and with limbs in 

 the form of swimming-paddles. The vertebrae are 

 amphicoelous. A sacrum is absent, so that only pre- 

 caudal and caudal regions are distinguishable. The 

 ribs have two heads for articulation with the ver- 

 tebrae : a sternum is absent, but there is a highly 

 developed system of abdominal ribs. The skull is 

 produced into an elongated rostrum, formed chiefly 

 of the premaxillae, and with small nostrils situated 

 far back. The orbits are large and contain a ring 

 of bones developed in the sclerotic. An epipterygoid 

 is present as in Lizards, and there is a large parietal 



foramen. Both superior and inferior temporal arches are developed. The 

 quadrate is immovably fixed to the skull. The pectoral arch consists of cora- 

 coid, scapula and clavicle, the pro-coracoitl being absent or very small. The 

 coracoids are broad bones which meet vent-rally for a short distance without 

 overlapping. A T-shaped episternum is present. The bones of the pelvis are 

 not strongly developed ; the ilia are not connected with the spinal column ; the 



VOL. II A A 



