SEGMENTS OF TRIGEMINAL NERVE-GROUP 



275 



service, and would form an internal and not an external group of 

 eye-muscles. 



In Fig. 110, A, taken from Miss Beck's paper, I have shown the 

 relative position of the eyes and the segmental dorso-ventral pro- 

 somatic muscles on the carapace of the scorpion. In Fig. 110, B, I 

 have drawn the prosomatic carapace of EurypteruS Scouleri, taken 

 from Woodward's paper, with the eyes as represented there ; in this 

 I have inserted the segmental dorso-ventral muscles as met with in 

 the scorpion, thereby demonstrating how, with the degeneration of 

 the median eyes and the large size of the lateral eyes, the recti 

 muscles of the scorpion would approach the position of an internal 

 recti group to the lateral eyes, and so give origin to the group of 

 muscles innervated by the oculomotor 

 nerve. In the Eurypterus these large 

 eyes are large single eyes, not separate 

 ocelli, as in the scorpion. 



All, then, that is recpiired is that in 

 the first formed fishes, which still pos- 

 sessed the dorso-ventral muscles of their 

 Eurypterid ancestors, the lateral eyes 

 should be the important organs of sight, 

 large and near the mid-dorsal line. Such, 

 indeed, is found to be the case. In 



— f 



... Occ 



Fig. 111. — Dorsal Head- 

 Shield of Tremataspis 

 Mickwitzi. (From Rohon.) 



amongst the masses of Eurypterids found Fr °-> narial opening; I.e., late- 



,, ,-,., . , . ~. , ral eyes; flZ.,glabellum plate 



in the upper Silurian deposits at Oesel, as ove / bram; 8 0cc<) occ f pital 



described by Eohon, numbers of the most spine. 



ancient forms of fish are found belonging 



to the genera Thyestes and Tremataspis. The nature of the dorsal 



head-shields of these fishes is shown in Fig. 14, which represents 



the dorsal head-shield of Thyestes verrucosus, and Fig. Ill that of 



Tremataspis Mickwitzi. They show how the two lateral eyes were 



situated close on each side of the mid-dorsal line in these Eurypterus- 



like fishes, in the very position where they must have been if the 



eye-muscles were derived from the dorso-ventral somatic muscles of 



a Eurypterid ancestor. 



In Lankester's words, one of the characteristics of the Osteostraci 



(Cephalaspis, Auchenaspis, etc.), as distinguished from the Fletero- 



straci (Pteraspis), are the large orbits placed near the centre of the 



shield. The apparent exception of Thyestes mentioned by him is no 



