MEDIAN EYES IN EXTINCT VERTEBRATES 343 



that the exoskeleton did not give rise to the endoskeleton, as has been 

 generally maintained, but that the exoskeleton and the endoskeleton 

 were formed simultaneously. 



Moreover, since Gaskell's time the general architecture of the Ostra- 

 coderm fishes has been studied with great exactitude by Stensio, Kaier, 

 and other Norwegian palaeontologists, who by the use of modern methods 

 of research have been enabled to study the markings on the interior of 

 the shields and obtain from serial sections reconstruction models repre- 

 senting complete casts of the cavity of the skull (Fig. 238, A and B, p. 341). 

 Wax models of the interior of the skull have been made in much the same 



Fig. 241. — Front of Head of Porthetis spinosus, Transvaal, showing the 

 three Frontal Ocelli, arranged in the Typical Triangular Manner, 

 One in front, Two behind. (Redrawn from Cambridge Natural History.) 



way as embryological material is reconstructed from serial sections by 

 the wax-plate method of Born. The reconstructions made from serial 

 sections of the fossil specimens are magnified sufficiently to allow of a 

 detailed study being made of the main cavities which enclosed the brain 

 and branchial system ; the minor cavities and canals which contained the 

 cranial nerves, venous sinuses, and cerebral arteries ; the afferent and 

 efferent vessels of the branchial apparatus ; also the orbital cavities ; 

 labyrinth ; naso-hypophyseal canal ; pineal recess ; and the canals for 

 the nerves and vessels leading to the impressions which Stensio describes 

 as the median " dorsal electrical field," and the marginal or " lateral 

 electrical fields " on the dorsal aspect of the shield, Fig. 228. 



From an extensive study of a very large amount of material obtained 

 from the Downtonian and Devonian strata of Spitzbergen and other 



