i;n 



little above tlic plane of the zygapophyses. Chevron-bones coossified! 

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

 nclle pierced in the parietal bone. 



Tiie sijecics of this genus are, so far as known, the most elongate iu the 

 order. The quadrate 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.propytlwn, to that 

 of the C. j>la?iif/otis, where they are narrowed posteriorly in some degree, to 

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

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

 mediate genus. The numl)er 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 tortou, Cope. 



A slender species of some thirty feet iu length, w ith a narrow, pointed 

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

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

 was armed with eleven teeth. 



The frontal hone is light, and witli 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, with 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 imrietal 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 postfrontal, and presents a transverse 

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

 longitudinal surtiice; the angles bordering the temporal fbssse being obtuse, 

 well se|)arated, and low. Bases of slender parieto-squamosal arches project 

 from it l)eliin(l. 



