ICHTHYOSAURIA. 393 



suprateniporal vacuity ; x space lor ectopterygoid (?) ; v vomer. Restored from specimen 

 in Britisli Museum. 



E Upper tooth of Ichthyosaurus communis showing enamelled crown and fluted base, 

 X i (L. Lias). 



F transverse section of base of tooth. 



G atlas and axis of Ichthyosaurus lon^i/rons, U. Lias, ft.occoccipital condyle of skull ; 

 c' centruir of atlas ; s- centrum of axis ; n' paired neural arch ot atlas ; >t^ single neural arch 

 of axis ; r ribs ; w subvertebral wedge-hones (hypocentra) (after Owen). 



H dorsal vertebra and ribs of Ichthyosaurus, anterior aspect, L. Lias, abd abdominal 

 rib ; r rib (after Owen). 



/, </, K vertebra from base of tail and two caudals of Ichthyosaurus, anterior aspect, ch. 

 chevron bone ; r rib (after Owenl. 



L section of caudal vertebra of Ichthyosaurus. 



M ventral view of pectoral girdle of Ichthyosaurus (restored from specimen in Brit. Mus.). 

 ac glenoid cavity ; cl clavicle ; co coracoid ; i.cl interclavicle ; sc scapula ; x edges originally 

 bordered by cartilage. 



N. Outer aspect of left pelvic arch of Ichthyosaurus communis (from specimen in Brit. 

 Mas.), il ilium ; is iscl ium ; pb pubis. 



can it be said that the descent is even approximately known. The pro- 

 gress of palaeontological research has made known to ns the immense 

 variety which has characterised the organisation of reptiles, but it is at 

 th^ same time rendering more difficult the elucidation of pedigrees. For 

 the greater the variety in organisation that is revealed to us, the more 

 involved become the interrelationships between the different groups. 



The skull (Fig. 218 yl-jD) has a long rostrum which consists almost en- 

 tirely of the premaxillae. The maxillae are small and the anterior nares 

 are placed far back just in front of the large orbits. The teeth which are 

 conical and crocodilian are placed in a continuous groove (rarely in 

 separate pits) on the premaxillae, maxillae and mandibles. They some- 

 times show a folding of the enamel as in Labyrinthodonts. The eye has 

 a circle of bony sclerotic plates and the orbit is closed behind by the post- 

 frontal, postorbital, and jugal. There is a single broad temporal arcade 

 which consists of squamosal, supratemporal (prosquamosal) and qviadrato- 

 jugal, and corresponds to the superior and inferior temporal arcades of 

 Rhynchocephalia etc., the lateral temporal fossa being closed. There is 

 a supratemporal fossa bounded by the parietal, sqvtamosal and post- 

 frontal. The nasals are large, the parietals and frontals are small and 

 paired ; there is a large pineal foramen between the parietals and frontals. 

 All the occipital bones are present and separate, as are the opisthotic 

 and prootic. The single occipital condyle is formed by the basioccipital 

 alone. There is a basisphenoid, but alisphenoids, presphenoid and orbito- 

 sphenoids are absent. There is said to be an epipterygoid reaching from 

 the pterygoid to the prefrontal. A median splint extends forwards from 

 the basisphenoid in the vacuity between the pterygoids ; this may be 

 called the parasphenoid or basisphenoidal rostrum. The pterygoids are 

 large and widely separate except in front where they touch ; behind they 

 join the basisphenoid and quadrate, and extend anteriorly between the 

 palatines to the vomers. The internal nares are between the vomers on 

 the inside and the maxillae and palatines on the outside. The quadrate 

 is fixed. The mandible is long and narrow and without a coronoid pro- 

 cess. The hyoid appears to be represented by a pair of stovit, rib- like 

 bones beneath the pterygoids. 



The vertebrae (Fig. 218 G-L) are very numerous, 150 or more in mmaber, 

 of which 100 are caudal. They are divisible into caudal and precaudal 

 only. The centra are amphicoelous and very short anteroposteriorly. 

 The neural arches are separate from the centrum and the zygapophyses 

 are feeble and sometimes even absent. The centra possess on each side 

 two short lateral processes, to which the double headed ribs are attached. 



