224 



REPTILIA class in 



the close junction of the astragalus with the tibia, or tendency to form a 

 tibio-tarsus, is an avian resemblance common to the majority of Dinosaurs. 



While, on the one hand, the cleft between Dinosaurs, crocodiles, and 

 lacertilians is bridged by the Rhynchocephalian Proterosaurs (Proganosauria, 

 IJaul), on the other it may be regarded as altogether probable that birds 

 have come down to us from a primitive (perhaps bipedal?) Dinosaur -avian 

 stem which sprang off from a Palaeohatleria-like ancestor in the Permian. 1 The 

 earliest known Dinosaurs (Theropoda) of Triassic age are shown by their limb 

 structure and footprints to have been mainly bipedal in gait, and by their 

 dentition to have been of carnivorous habits; of the herbivorous Dinosaurs 

 (Sawopoda and Predentata) which range throughout the Jura and Cretaceous, 

 some must have been bipedal and others quadrupedal. The culmination of 

 Dinosaurs occurred during the Upper Jura and Cretaceous, in the interval 

 between the decline of Theromorph reptiles and the dominance of the 

 Mammalia. During their period of ascendency numerous hypertrophic 

 forms were evolved, which attained proportions far exceeding those of all 

 other known terrestrial creatures. Over-specialisation having run its course, 

 a rapid decline and extinction followed. 



A bony exoskeleton is developed to some extent among certain of the 

 Theropoda and Predentata, but in the majority of Dinosaurs the skin was 

 either naked or protected by horny scales. The dermal armour, when 

 present, consists either of isolated bony plates or spines, or of interlocking 

 scutes which form a continuous shield encasing portions of the trunk and tail. 



The vertebrae are usually either amphiplatyan or opisthocoelous, more 

 rarely amphicoelous. There are nine to fifteen cervicals, ten to eighteen 

 dorsals, usually from two to six, but sometimes as many as ten, fused sacrals, 

 and thirty to fifty caudal vertebrae. The union between the centra and 

 their neural arches is commonly by suture. In the anterior cervicals the 

 neural spines are generally short or rudimentary, and gradually increase in 

 size toward the thoracic region. The atlas and axis, so far as known, exhibit 

 a construction similar to that in crocodiles. All the succeeding cervicals bear 

 double-headed ribs, of which the capitulum is attached to the parapophysis of 

 the centrum, and the tuberculum to the transverse process of the neural arch. 

 In the dorsal region the parapophyses mount upward from the centra on to 

 the sides of the neural arches. Single-headed ribs do not occur. 



Among the Theropoda and Sauropoda the posterior dorsal vertebrae articu- 

 late with one another not only by zygapophyses, but by a hyposphene- 

 hypantrum arrangement. The hyposphene facette is a vertical or wedge-shaped 

 projection occurring on the posterior end of the neural arch below and 

 continuous with the post-zygapophysis. It is received into a corresponding 

 groove, or hypantrum, on the anterior face of the next vertebra behind. This 

 arrangement is similar in function to the zygosphen-zygantrum articulation 

 among Ophidians, except that the relative positions of pegs and sockets are 

 interchanged. The sacral vertebrae, which vary from two to ten in number, 

 ;ue fused together; and the chevron bones of the caudals are articulated 

 intervertebrally. 



The skull of most Dinosaurs is extremely small in proportion to the rest 

 of the body, more so in Brontosanras, in fact, than in any other reptile, and 



Osbom, II. /•'.. Reconsideration of the evidence for a common Dinosaur-avian stem in the 

 Permian (Amer. Nut. vol. XXXIV. p. 777), 1900. 



