MEDIAN EYES IN EXTINCT VERTEBRATES 



345 



in support of Stensio's conclusion that the Myxinoidea and Petromyzontia 

 have undergone regressive changes in their descent from the ostracoderm 

 ancestral stock. 



Now in the Downtonian strata of Ringerike, about 35 miles north- 

 west of Oslo, there have been found, according to the description given 



Fig. 242. 



-Front of Head of Copiophora cornuta (female). 

 Locustid^e. 



Demerara — 



There is a high degree of development of the median frontal eye which forms 

 a conspicuous feature of the Frons ; the other two ocelli are poorly developed 

 and are placed one on each side of the curious frontal cone. (Redrawn from 

 Cambridge Natural History.) 



by Kaier, large quantities of the remains of fossil crustaceans belonging 

 to the order Merostoma (e.g. small and giant Eurypterids). In close 

 proximity to these, embedded in a stratum termed by him the " fish 

 horizon," are found numerous examples of the Ostracodermi and more 

 particularly of the groups Anaspids, Cephalaspidae, Ccelolepidee, the 

 head shields of which are represented in a schematic manner in Fig. 232. 

 Although the general exoskeleton of the Anaspidae consists mainly of 



