322 



CLASSIFICATION OF VERTEBRATES. 



united by ligament at the symphysis. Sternum and episternum but rarely 

 occur, the coracoids (which bear a procoracoid process) meeting in the 

 middle line. The pelvis was weak, and in most 

 forms the ilium was without connection with the ver- 

 tebral column. The bones of the limbs were short. 

 In most forms the skin was naked, or at least lacked 

 dermal ossicles. 



Two groups are recognized, the PLIOPLATE- 

 CARPID.-E, with a sacrum of two fused vertebrae and 

 an episternum ; and the MOSASAURID^E, in which a 

 sacrum was lacking. Plioplatecarpus occurs in the 

 rocks of Maestricht. Mosasaurus (species of which 

 occur in Holland, England, and the eastern U. S.) 

 was first found over a hundred years ago. Clidastes 

 from Alabama, and Platecarpus from Kansas, are 

 pretty well known. Liodon, from the cretaceous of 

 both continents. Some of the phythonomorphs were 

 over 40 feet in length. 



FIG. 316. Skull of 

 Liodon, after Owen. 

 b, basioccipital ; /, 

 frontal ; j, jugal ; /, 

 lachrymal ; mx, max- 

 illa; w, nasal; /, 

 parietal, with large 

 parietal foramen ; 

 pm, premaxilla ; pf, 

 prefrontal ; pof, post- 

 frontal ; </, quadrate ; 

 s, squamosal ; st, su- 

 pratemporal. 



SUB-ORDER 3. OPHIDIA (SERPENTES). 



Footless, elongate, scaled reptiles, with proccelous 

 vertebrae ; without chevron bones, sacrum, or pectoral 

 girdle ; no parietal foramen or temporal arch ; sternum 

 and tympanum lacking ; no movable eyelids ; tongue 

 bifid, protrusible ; teeth acrodont ; no dermal osseous 

 scutes ; no urinary bladder. 



Snakes are to be confused only with the footless 

 lizards, from which they differ, however, in many 

 structural features. The body is covered above by 

 imbricate scales, while the lower surface of the body 

 is usually covered by broad plates, the abdominal scutes. The scuta on 

 the head are regularly arranged, and the characters they present are of 

 value in classification (Fig. 317). These scales are regularly shed, and as 

 regularly reformed by epidermal cornification. 



The procoelous vertebrae, which may exceed 400 in number, bear zygan- 

 tra and zygosphenes, and can only be divided into caudal and precaudal 

 series. The anterior 10 to 30 bear large hypapophyses. The neural 

 arches are fused to the centra, and the ribs, which are frequently hollow, 

 begin with the third vertebrae. Abdominal ribs and all sternal structures 

 are lacking. 



The bones of the skull are very solidly ossified, the brain capsule being 

 long, and closed in front. Many of the bones (maxilla, pterygoids, supra- 

 temporal) are loosely articulated together. The parietals are laterally elon- 

 gate, and fused with the prootic, ali- and orbitosphenoid. The opisthotics 



