131 



little above the plane of the zygapophyses. Chevron-bones coossified ■ 

 cervical hypapophyses free. Humerus short and very wide. Parietal fonta- 

 nelle pierced in the parietal bone. 



The species of this genus are, so far as known, the most elongate in the 

 order. The cpiadrate bone, in the most typical species, exhibits a very 

 prominent internal longitudinal ridge, indicating a great degree of rotation 

 of that bone, and hence external flexure of the mandibular rami. 



There are specific differences in the form of the palatine bones in this 

 genus, from the more transverse or expanded type of the C.propython, to that 

 of the C. planifrons, where they are narrowed posteriorly in some degree, to 

 the C. tortor, where they are vertically placed in the posterior half. Clidastes 

 is nearly allied to Platecarpus, with which Sironectes associates it as an inter- 

 mediate genus. The number of species already known is considerable, and 

 the genus is divided into sections, for convenience of reference : 



A. Centra of dorsal vertebrae depressed: 



a. Frontal bones with median keel: 



Clidastes tortor, Cope. 



A slender species of some thirty feet in length, with a narrow, pointed 

 head of two and a half feet. Its teeth are compressed, andwith a cutting- 

 edge fore and alt, and were eighteen in number on the under jaw; the palate 

 was armed with eleven teeth. 



The frontal bone is light, and with thin margins ; it is keeled above for 

 the anterior .three-fourths of its length. On its inferior face, the olfactory 

 groove is closed by the apposition of its lateral bounding ridges. Posterior 

 to this point, the latter diverge and disappear; and a median ridge, witli an 

 acute edge on each side, carries the deep median olfactory groove, but disap- 

 pears with the lateral ridges. There is no grooved triangular area in front 

 of the foramen parietale. The parietal bone below presents a V-shaped, 

 rounded ridge; the limbs embracing the narrow and small parietal fontanelle 

 in front. The bone, in general, is broad and expanded laterally. Its antero- 

 lateral ala is largely underlaid by the post-frontal, and presents a transverse 

 ridge for its boundary. The superior surface of the bone presents a flat, 

 longitudinal surface; the angles bordering the temporal fosste being obtuse, 

 well separated, and low. Bases of slender parieto-squamosal arches project 

 from it behind. 



