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 required 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, 
i : Fie. 111.—Dorsan Hzap- 
large and near the mid-dorsal line. Such,  gurmrp oF Tremataspis 
indeed, is found to be the case. In Mickwitzi. (From Roxon.) 
amongst the masses of Eurypterids found "re. narial opening; L.e., late- 
z oes 7 ral eyes ; gl., glabellum plate 
in the upper Silurian deposits at Oesel,as = 4... pean Ole eoaipitel 
described by Rohon, 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. 111 that of 
Tremataspis Mickwitz. 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 Hetero- 
straci (Pteraspis), are the large orbits placed near the centre of the 
shield. The apparent exception of Thyestes mentioned by him is no 
