PHYLUM CHORDATA 



373 



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



ventral portions (acromial processes) of the scapulie 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. In 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 



Fia. lOU. — PlesiosauruSj pelvic arch. li. ilium ; Js. iscliium ; 

 Pb, pubis. (After Huxley.) 



(0 



o 

 >> 



elongated snout, no neck, and an elongated tail, 



with a large vertical caudal fin, and with limbs in ; ^SK/rail* i 



the form of swimming-paddles. The vertebras are "~ 



amphiocelous. 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 premaxillee, 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. T]ie 

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

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

 coracoids are broad bones which meet ventrally 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 colmnn ; the 



VOL. IE 



A .V 



