CHAP. X DINOSAURIA SKELETON 413 



abolished. Teeth, mostly alveolar and laterally compressed, are 

 restricted to the dentary, maxillary, and premaxillary bones. 

 In the Orthopoda the latter carry no teeth, or these are restricted 

 to the lateral portion, leaving a wide diastema. This toothless 

 part plays upon a peculiar crescent-shaped bone, the so-called 

 predentary, which rests loosely upon the anterior ends of the' 

 mandibular rami, which latter do not as a rule form an 

 osseous symphysis. The Ceratopsia possess in addition a similar 

 upper toothless piece, the prerostral, a kind of pre-premaxilla. 

 The morphological value of these extra pieces is quite obscure ; 

 they were in all probability provided with thick, horny pads. 

 The bones of the roof of the mouth recall in their arrangement 

 that prevailing in the Ehynchocephalia and the Parasuchia. 

 There are two pairs of large vacuities ; one between the maxillae, 

 ectopterygoids and palatines ; the other between the latter, the 

 maxillae and the usually small or slender vomers. The pterygoids 

 are perhaps the largest bones, and form a rather long symphysis ; 

 laterally and behind they abut against the quadrate, anteriorly 

 against the ectopterygoids and the palatines, which latter they 

 sometimes separate. A peculiar feature of some skulls, e.g. 

 Ceratosaurus and Triccratops is the great size of the groove in 

 which the large hypophysis of the brain is lodged. 



The vertebrae are very variable, amphicoelous, opisthocoelous, 

 nearly plain, with a slight concavity behind, or occasionally 

 procoelous in the anterior region of the tail. Besides the usual 

 pre- and post-zygapophyses many Sauropoda and Theropoda 

 possess on the posterior trunk-vertebrae additional joints, effected 

 by a vertical wedge, the hyposphene, which extends backwards 

 from between the post-zygapophyses and fits into a notch 

 between and below the anterior zygapophyses of the next follow- 

 ing vertebra. These additional articulations are analogous to 

 the zygosphenes and zygantra of snakes and iguanas, except that 

 in these Sauria the wedges are formed on the opposite, namely 

 the anterior ends of the vertebrae. The vertebrae of the neck 

 and trunk are devoid of intercentra, but those of the tail carry 

 long chevron-bones. The number of sacral vertebrae is generally 

 increased to four or five. The ribs have well-developed capitula 

 and tubercula, and the former have the tendency to shift from the 

 centra or from their parapophysial processes on to the usually 

 much elongated diapophyses of the neural arches. This arrange- 



