XIIl 



PHYLUM CHORDATA 



.373 



ahlo features. Tlie coracoids jiUmi^s meet in a ventral Myiiipli} sis, ami tlie 



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



is, in most cases, an arch of lione, consisting of a moilian and two lateral 



Ijortions, wiiicli probaldy represent tiie episternum an<l the clavicles : in sonic 



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



meet in the middle line : the two symphj'ses may 



remain separate (Fig. 1014), or tliey may unite so as f^! 



to divide the space into two separate obturator 



foramina. The teeth are implanted in distinct 



sockets. 



The Sauropterygia date back to tiie Trias, and 

 perhaps to tlie i'eimian, extending onwards to the 

 Cretaceous. 



TcllTllVOPTERYaiA. 



Tlie Ichtliyoptei'ygia, including Ichthyosaurus (Fig. 

 lOI,")) and its allies, were aquatic Reptiles, some of 

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

 what fish-like body, large head produced into an 



J^O. 



Fig. 1014.— Plesiosaurus, pelvic arch. It. Ilium ; Is. ischium ; 

 I'b. pubis. (After Iluxlcy.) 





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elongated snout, no neck, and an elongated tail, 

 with a large vertical caudal fin, and with limbs in 

 the form of swimming-paddles. The vertebra are 

 aniphictelous. A sacrum is absent, so that only jire- 

 caudal and caudal regions are distinguishable. The 

 ribs have two heads for articulation with the ver- 

 tebra; : a sternum is aljsent, but there is a highly 

 developed system of abdominal ribs. The skull is 

 produced into an elongated rostrum, formed chiefly 

 of the premaxill;¥, and with small nostrils situated 

 far back. The orbits are large and contain a ring 

 of bones developed in the sclerotic. x\n epipterjgoid 

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



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

 quadrate is immovablj' fixed to the skull. The pectoral ai'ch consists of cora- 

 coid, scapula and clavicle, the pi'o-coracoid being absent or very small. The 

 coracoids are broad bones which meet ventrally for a short distance without 

 overlappmg. 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. H A A 



