312 CLASSIFICATION OF VERTEBRATES. 



ORDER IV. ICHTHYOSAURIA (ICHTHYOPTERYGIA). 



Extinct aquatic reptiles with naked skin, large head, short 

 neck, long bilobed tail and flipper-like appendages. Amphi- 

 coelous vertebrae ; no sacrum ; vertebral column extending into 

 lower lobe of tail ; no sternum ; ribs bicipital, abdominal ribs 

 present; quadrate immovable ; jaws long and pointed, the upper 

 jaw composed chiefly of premaxillae ; teeth usually numerous 

 (absent in Raptaiwdori) and seated in a common groove. 



FIG. 308. Skull of Ichthyosaurus, after Zittel. a, angulare; d, dentary; 

 j, jugal ; /, lachrymal ; 7/uc, maxillary ; , nostril ; na, nasal ; pa, parietal ; pf t post- 

 frontal ; pnt, premaxillary ; po, postorbital ; p, prefrontal ; qj, quadratojugal ; s, 

 squamosal ; sa, supraangulare ; st, stapes. 



The neural arches of the vertebrae are united by suture to 

 the centra ; the caudal vertebrae have chevron bones ; supra- 

 temporal fossa and parietal foramen are present. The orbits 

 are very large, and contain a ring of sclerotic bones ; the exter- 

 nal nostrils are just in front of the orbits. The prefrontals are 

 as large as, or larger than, the frontals ; the pterygoids extend 

 forward between the palatines to the vomers, and a large para- 

 sphenoid is present. 



The coracoids meet in the middle line ; procoracoids are 

 lacking. The pelvis is entirely free from the vertebral column, 

 and its elements are reduced. The limbs are very short and 

 paddle-shaped, the radius, ulna, tibia, and fibula being reduced 

 to polygonal bones, distinguishable only by position from the 

 metapodial elements. The digits are usually five, but this num- 

 ber is sometimes apparently increased either by fission or by 

 formation of marginal rows ; the phalanges are very numerous. 



