120 REPTILES 



by means of a peg-and-socket arrangement — a structure 

 paralleled in some of the extinct armour-clad ganoid fishes. In 

 the earlier family Teleosauridcz, on the other hand, the dorsal 

 armour consists of two longitudinal rows of rounded plates ; 

 and the ventral buckler is divided into an anterior and a pos- 

 terior portion, in which the component plates (each formed by 

 a single piece) of the hind transverse rows are united by suture. 

 The dorsal buckler of Pelagosaurus affords an admirable 

 illustration of the type characteristic of the TeleosauridcE. 



From the foregoing comparisons it will be apparent that 

 while in the anterior region of the ventral buckler of all croco- 

 diles in which this is developed the component plates of each 

 transverse row articulate with one another by a sutural union, 

 in the posterior moiety of this buckler in the Jurassic forms the 

 articulation of the transverse rows may be either by suture, or 

 by imbrication. Here, again, we are unable to account for 

 these structural differences. 



Finally, as mentioned elsewhere, in the marine Jurassic 

 crocodiles of the genera Geosaurus, Dacosaurus, and Metrio- 

 rhynchus the dermal armour, in adaptation to the needs of a 

 pelagic existence, has been discarded. 



In the belodonts of the Trias, comprising the genera Phyto- 

 saurus, Stagonolepis, and Parasuchics, which are referred by 

 some authorities to the Crocodilia, while by others they are 

 regarded as representing an order (Parasuchia) by themselves, 

 there was also a pitted dermal armour very similar to that of 

 the true crocodiles. In all the genera the dorsal plates are 

 keeled and form two longitudinal rows with a few smaller 

 lateral series ; while those of the ventral armour, which is pre- 

 sent in Stagonolepis but absent in Phytosaurus, are arranged in 

 not more than eight longitudinal rows, with each plate consisting 

 of only a single piece. Whether we regard the dermal armour 

 of the belodonts and of the true crocodiles as inherited from a 

 common ancestor, or independently developed in each case, 

 must largely depend upon the view taken as to the affinity of 

 the two groups. 



Other crocodile-like reptiles from the Trias, such as Aeto- 

 saurus, with pitted dermal armour, were apparently more or less 

 intimately related to the belodonts ; but it is at present impos- 

 sible to say whether their armature has been derived from a 



