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SCIENCE 



[N. S. Vol. XXXVI. No. 922 



virtually senile origin, have plainly been the 

 primary factor in giving to the Testudinata 

 an exceedingly long lease of life. 



Appositely, in the Dinosauria, a far more 

 active and aggressive race, strong develop- 

 ment of both body and cranial armature, in 

 both the upper and the nether dermogene lay- 

 ers, largely confines itself to the bizarre pat- 

 terns of Cretaceous times. Thus, it is plainly 

 the under layer which gives rise to the huge 

 plate roofing the entire skull in the remark- 

 able genus Anhylosaurus of Brovsm. In the 

 Ceratopsids, as stated, the outer dermogene 

 layer forms the epoccipital fringe of the under 

 stratum, vchich is not, as at first supposed, an 

 excrescent skull growth, but deep dermal bone 

 in reality strictly homologous to the hip arma- 

 ture of Polacanthus. Considered separately, 

 we can reach but indefinite surmises as to the 

 mode of origin or the meaning of these armor 

 features. But clearly, when taken in their 

 complementary relation, unity is restored to 

 the armored series, and the simple structure 

 generalization which clears up its true nature 

 is at once discerned. 



In a word, then, the Dinosaurs, instead of 

 eventually confining extensive dermal develop- 

 ment 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 ossifica- 

 tions of essentially senile nature or origin. 

 In either case, in strong contrast to the con- 

 servative armor development seen in the 

 turtles, this growth of the most formidable 

 armature known in land animals must have 

 resulted in a most delicately balanced envir- 

 onmental adjustment in the entire race of 

 armored Dinosaurs. 



Obviously, too, this conception of the Dino- 

 saurian armor as arising from the two dermo- 

 gene bone-forming layers is still 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 form- 

 ing the most ornate patterns. The plate, or 

 flat dermal element, thus lifts itself up by the 

 simplest process into the great frill of Tri- 

 ceratops, the tremendous erect flat plates of 

 Stegosaurus, or the huge caudal spines of the 

 latter animal or of Nodosaurus or those of 

 Hierosaurus. Furthermore, the development 

 of the supracranial horn-cores in Triceratops 

 can, whatever their origin, offer no difficulty 

 to the parallel between Dinosaurian and Tes- 

 tudinate armature here drawn, since these 

 features are at least morphologically repeated 

 in Meiolania. In both these cases, too, the 

 horns may be viewed as exceptional struc- 

 tures quite apart from the dermal growth and 

 modifications characteristic of turtles, and 

 now known to have been present in an im- 

 mensely varied and cosmopolitan series of 

 Dinosaurians. The summation we therefore 

 fairly reach is that the growth impulse in the 

 dermogene layers which forms the pre- or 

 dermo-dentary diagnostic of the Predentata, 

 culminates in the rostral, dermocornutal and 

 frill investiture of the Ceratopsids; while the 

 dorsal armor of the Stegosaurs, and the 

 cranio-dorsal armature of the Nodosaurida 

 are all structurally homologous — it being in 

 most cases plain to which of the two dermo- 

 gene bone-producing layers any given element 

 belongs, just as in the Testudinata. 



In closing, I 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 groups, and invites renewed study 

 for the purpose of determining what endo- 

 skeletal variations resulted secondarily to the 

 development of the dermal armor. It encour- 

 ages us to believe, moreover, that the day can 

 not be far distant when some of the proximate 

 causes of armor development may be dis- 

 cerned, now that we see that armored Dino- 

 saurs are by no means so strangely or funda- 

 mentally different from other Dinosaurs or 

 even from other reptiles, as was once sup- 

 posed. G. E. WiELAND 



