472 



THE PINEAL ORGAN 



of lime-bearing tissue, intermediate between true bone and calcified 

 cartilage. The endoskeleton in the specimen described did not 

 participate either in the axial articulation or in the articulation between 

 the two segments of the fin, since both these articulations were 

 formed solely by the dermal bones. In the proximal articulation the 

 inner ends of the dermal bones embraced the neck, of the processus 

 brachialis of the anterior ventro-lateral plate ; the opening in the dermal 

 bones of the appendage which surrounded the process is called the axial 

 foramen, and besides enclosing the head or condyle of the processus 

 brachialis transmitted vessels and nerves to the appendage. The inter- 

 mediate position in a direct line of descent between fishes and the inverte- 

 brate Merostomata, which was claimed for the Antiarchi (Bothriolepis, 



Pterichthys), is thus not confirmed by 

 recent work. This Order being now 

 considered to belong definitely to the 

 fishes, their appendages being true 

 pectoral fins and their resemblance to 

 the large paddle-like appendages of 

 the Merostomata (Eurypterus or 

 Pterygotus) being functional rather 

 than structural. 



9. The theory that a higher race 

 of animals which was " predominant " 

 arose directly from a lower race in 

 the geological period which immediately preceded it, must, in the light 

 of modern knowledge, be radically modified, since it is evident that the 

 common ancestor of two highly differentiated and in many respects 

 divergent classes must have been of a much simpler type than either of 

 the two classes under consideration. Moreover, the divergence of the 

 two classes must have taken place long before dominance of one class 

 over the other could have existed as a factor in their evolution. 

 Certain points of similarity in particular organs or systems seem to 

 have been preserved in the two divergent classes, although even these 

 when critically examined are found to present modifications in detail ; 

 and the modifications or divergences are in general more pronounced 

 in the phylogenetically older races and in adult animals as compared with 

 their larval or embryonic stages. To take a concrete instance, the differ- 

 ence between the compound faceted eye of an arthropod and the inverted 

 eye of an adult vertebrate is very great, and since the upright faceted eyes 

 of certain arthropods were already highly evolved in some trilobites which 

 were living in the Upper Cambrian period and since median eye tubercles 

 have been found in both larval and adult specimens of Trinucleus and 



Fig. 321. — Intracranial Surface 

 of postmedian plate of 

 asterolepis ornata. (after 

 Stensio.) 



