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region. And similar usuai ly keeled series of various other Dinosaurs must also be 

 homologous to the Dermochelan mosaic. 



Only in the Stegosawus is it difficult to state whether the two huge rows of dorsal 

 spines belong to the outer, or to the nether dermogene armor-producing layer; while 

 it is not absurd to suggest that the upright dorsal plates could possibly result from 

 the fusion of elements of both layers. Though we should not lose sight of the alternative 

 explanati on that the skull piate of Ankylosaurus the horns and frill of Triceratops, 

 the dorsal plates of Stegosaurus and the dermo-iliac elements of Polacanthus. Nodosaurus 

 and Stegopelta may ali be homologous elements of a dominant midline armor arising from 

 the deep dermogene layer and thus in part analogous to the pleuralia of turtles. 



Similarly, going much further afield, it is entirely possible that in the origin of the 

 extraordinary supra-occipital crest of Pteranodon brought to light in the course of the 

 admirable studies of Eaton, some strictly dermal element has played a part. And, indeed, 

 responsive or counter growth of the endoskeleton finally resulting in fusion with dermogene 

 elements, with the complete obliteration of suturai lines, is of common observation in 

 the Vertebrata, being essentially a senile course of development, which has to do with 

 the aging of races qui te as much as the production of purely protective features. The 

 studies of Beecher on the origin of spines have interest in this connection. 



Going back to the first premise : As noted, in strong contrast to Crocodile-like 

 reptilian types which mainly develop the outer dermogene bone-producing layer the 

 turtles originali}' had prominently developed both the outer and nether dermogene layers. 

 But they tended early to strengthen and use the under layer only along very conservative 

 lines and never in their history developed cranial armature of any kind, save in the 

 comparatively recent and altogether aberrant Meiolania. On the contrary, as fusion of the 

 dermogene with the chondrogene elements of the carapace and plastron went on, the 

 armorless head became more or less retractile ; while the carapace and plastron, though 

 of virtually senile origin, have plainly been the primary factor in giving to the Testu- 

 dinata an exceedingly long lease of life. 



Appositely in the Dinosauria, a far more active and aggressive race, strong devel- 

 lopement of both body and cranial armature, in both the upper and the nether dermogene 

 layers, largely confines itself to the bizarre patterns of Cretaceous times. 



Thus, it is the under layer which gives rise to the huge piate roofing the entire 

 skull in the remarkable genus Anchylosaurus of Brown. In the Ceratopsids, as stated 

 the outer dermogene layer forms the epoccipital fringe of the under stratum, wich is 

 not, as at first supposed, an excrescent skull growth, but deep dermal bone in reality 

 strictly homologous to the hip armature 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, wheu taken in their complementary relation, unity is restored to 

 the armored series, and the simple structure generalisation which clears up its true 

 nature is at once discerned. 



In a word, then, the Dinosaurs, instead of eventuali}- confining extensive dermal 



