57 2 



The Class Ostracophori 



actual conditions of the dermal denticles of the ancestral Gnatho- 

 stome, as were unquestionably the dermal plates of Teleostomes 

 and Dipnoans. It seems equally if not more probable, on 

 the other hand, that the dermal armoring of the distinct groups 

 may have had an altogether different mode of origin, the product 



FIG. 356. The Horseshoe Crab or King-crab, Limulus polyphemus Linnaeus. Sup- 

 posed by Professor Patten to be an ally of the Ostracophores; usually re- 

 garded as related to the Spiders. 



of a crude evolution which aimed to strengthen the skin by a 

 general deposition of calcareous matter throughout its entire 

 thickness. The tuberculation of plates thus acquired might 

 have become an important step in the development of a more 

 superficial type of armoring which is most preferably represented 

 by the dermal denticles of Selachians. Nor, in passing, need 

 the presence of a mucus-canal system in the early plated forms 

 be of greater morphological importance than a foreshadowing 

 of the conditions of Gnathostomes, for this system of organs 



