ADAPTATIONS 121 



stock which gave rise to crocodiles, or to crocodiles and belodonts 

 together, or whether it is an independent development. In a 

 remarkable reptile from the Trias of America known as 

 Typothorax the pitted bony plates are said to be adherent to 

 the ribs, and it has been suggested that this fore-shadowed the 

 carapace of the Chelonia. The wide skeletal differences be- 

 tween tortoises and turtles on the one hand, and crocodile-like 

 reptiles on the other, demonstrates, however, that such a feature 

 cannot be regarded as indicative of the origin of the chelonian 

 carapace, and that, if really existing, it must be a special de- 

 velopment. 



Despite the marked structural resemblances of the extinct 

 dinosaurs to the primitive crocodile-like reptiles such as the 

 belodonts, it is evident that the dermal armaments developed in 

 many members of the former group have been evolved inde- 

 pendently of those of the latter. The armour of some of these 

 huge reptiles was of a most effective and often of a most bizarre 

 type. Some of the later representatives of the group in which 

 it occurs also developed large horn-like processes on the 

 skull comparable to those of certain extinct Tertiary mam- 

 mals such as the Dinocerata and Uintatheria ; such a de- 

 velopment of horn-like structures in the later members of a 

 group being apparently a sign of over-specialisation and 

 impending extinction. 



The armoured dinosaurs, in contradistinction to the 

 horned section, form a group (Stegosauria) nearly allied to the 

 iguanodons. The earliest known representative seems to be 

 Scelidosaurus harrisoni from the Lower Lias of Lyme-Regis, 

 Dorsetshire. In this species, which was about the size of an 

 average crocodile, although with a small head, there appear 

 to have been two longitudinal rows of keeled bony plates 

 running from the neck along the back, and converging into a 

 single row on the upper surface of the tail ; while the sides and 

 flanks were defended by numerous rows of smaller plates. 

 Hylcsosaurus, of the Sussex Wealden, seems to have been a 

 nearly allied although much more imperfectly known type, in 

 which the plates along the back were developed into long and 

 laterally compressed spines ; the precise mode of arrangement 

 of these spines being still unknown, although it was probably 

 very similar to that obtaining in Polacauthns foxi of the Wealden 



