160 GEOLOGICAL DISTKIBUTIOK. 



The most remarkable of all the Triassic reptiles are the Dmo- 

 sauria, a group of the greatest importance when viewed from a tele- 

 ological standpoint by reason of the many structural characters 

 which separate them from the typical reptiles, and approximate 

 them to birds. These avian characters are indicated principally in 

 the structure of the powerful pelvic girdle and hind limbs, which 

 depart very broadly from the normal type of reptilian structiire. 

 Thus, the pubic bones, in many cases, instead of projecting for- 

 wards as in other reptiles, are directed backwards, more nearly 

 parallel with the ischium, both bones therefore taking a position 

 directed towards the posterior portion of the body, a feature char- 

 acteristic of birds. In the hind limb, again, the ornithic char- 

 acters are seen in the great cnemial ridge which is developed on 

 the tibia, the gradual diminution of the fibula towards the distal 

 extremity, the structure of the astragalus, and in the disposition of 

 the digits, three or more, and their accompanying phalanges. The 

 inner and outer digits are shorter than the rest, or quite rudi- 

 mentary, and the third toe, as in birds in general, is the longest. 

 There are good grounds for concluding that the bones of the limbs, 

 and, doubtless, if this was the case, of some of the other portions 

 of the trunk, were permeated with air-passages, as in birds. The 

 structure of the fore limbs is still only imperfectly understood, 

 but there is no doubt that in many cases they were but very 

 feebly developed, being very much shorter than the hind limbs, 

 and that progression was, either habitually, or at least at times, 

 efEected by means of the posterior appendages alone. The remains 

 of these earliest dinosaurs are indicated both by the actual parts 

 pertaining to the skeleton (Zanclodon, Thecodontosaurus, Amphi- 

 saurus, Clepsysaurus, Bathygnathus), and by the foot-prints, many 

 of them three-toed, that have been left implanted in the rock- 

 masses. Some of these, which measure fully a foot, or even con- 

 siderably more, in length, were originally supposed to represent the 

 imprints made by the feet of giant birds, a suspicion strengthened 

 by the serial arrangement in twos in which the tracks are disposed; 

 but now that the structure of the dinosaurs has been more accu- 

 rately determined, and their ornithic characters and mode of pro- 

 gression recognised, there can be little doubt that they represent 

 the imprints of the reptiles belonging to this order. This view is 

 further strengthened by the circumstance that no actual remains of 



