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development to a single nether layer covering the body region only, as in the turtles, 

 tended to develop both the nether and outer layers in the body or skull or both. And 

 this is only another but definite way of saying that the dermal armature was variously 

 developed in the Dinosauria, or that it tended to assume bizarre patterns, whether we 

 consider the final results as devices for offense or defense, or a primary or secondary 

 use of dermal ossifications of essentially senile nature or origin. In either case in strong 

 contrast to the conservative armor development seen in the turtles, this growth of the 

 most formidable armature known in land animals must have resulted in a most deli- 

 cately balanced environmental adjustment in the entire race of armored Dinosaurs. 



Obviously, too, this conception of the Dinosaurian armor as arising from the two 

 dermogene bone-forming layers is stili further simplified on observing the Constant 

 tendency of the separate plates or elements to develop nodes of growth, which 

 could arise anywhere on their surfaces or borders, in series forming the most 

 ornate patterns. The piate, or fiat dermal element, thus lifts itself up by the simplest 

 process into the frill of Triceratops, the tremendous erect fiat plates of Stegosaurus, 

 or the huge caudal spines of the latter animai or of Nodosaurus, or those of Hierosaurus. 



Furthermore the development of the supracranial horn-cores in Triceratops can offer 

 no difficulty to the parallel between Dinosaurian and Testudinate armature here drawn, 

 since these features are at least morphologically repeated in Meiolania. 



In both these cases, too, they horns may be viewed as exceptional structures quite 

 apart from those dermal growths and modifications characteristic of turtles, and now 

 known to have been present in an immensely varied and cosmopolitan series of Dino- 

 saurians. 



In closing 1 may be allowed to assert that this exceedingly simple explanation of 

 the Dinosaurian armor at once gives us a clearer conception of the relationships of the 

 various Dinosaur groops, and invites renewed study for the purpose of determining what 

 endoskeletal variations resulted secondarily to the development of the dermal armor. 

 It encourages us to believe, moreover, that the day can not be far distant when some 

 of the proximate causes of armor development may be discerned, now that we see that 

 armored Dinosaurs are by no means so strongely or fundamentally different from other 

 Dinosaurs or even from other reptiles as was once supposed. 



The summation we therefore fairly reach is that the growth impulse in the der- 

 mogene layers which forms the pre- or dermodentary diagnostic of the Predentata of 

 Marsh, culminates in the rostral, dermocornatal, and frill investiture of the Ceratopsids ; 

 while the dorsal armor of the Stegosaurus and the cranio-dorsal armature of the 

 Nodosauridae are ali structurally homologous, it being in most cases plain to which of 

 the two dermogene bone -producing layers any given element belongs just as in the 

 Testudinata. 



The bones which Hay called fascia bones, the gastrals of crocodilians, the pternals 

 of turtles and other reptiles, must ali in our judgement originate in the deeps dermogene 

 layer. Such elements cannot therefore be in any final sense « subdermal » merely 



