42 NEW YORK STATE MUSEUM 
that the eye of Eurypterus had advanced one step beyond that of 
Pterygotus leading to the separation of the lens cone into an independent 
- ervstalline cone, and that this crystalline cone had failed to become 
hardened. In other arthropods, as in such insects as form Grenacher’s 
‘“aconous type’’ of the compound eye, the whole ommatidial cell may 
remain as a clear transparent body [Watase, zbzd]. | 
The bearing of the position of the compound and simple eyes on the 
habits of life of the eurypterids will be noted in another chapter. 
The median eyes or ocelli, which are also frequently termed the larval 
or stmple eyes, consist of two single, transparent spots of the integument 
in round pits with ringlike walls situated on a small mound or tubercle. 
In Eurypterus and Eusarcus the transparent spots are distinctly thickened 
into lenslike bodies. The ocellar tubercle is always situated on the median 
line of the carapace and it wanders forward and backward with the lateral 
eyes so that as a ruleit is on a cross line connecting the posterior extremities 
of the lateral eyes. Exceptions are made by the genera Eusarcus,. Ptery- 
gotus and Slimonia where the lateral eyes have become marginal at the 
front of the carapace while the ocelli remain on the middle of the carapace, 
thus retaining the advantage of their position at the apex of the shield. 
In Limulus, on the contrary, the lateral eyes have remained in the pos- 
terior half and the ocelli have wandered to the front. 
As to the function of the ocellt and compound eyes, observations on 
other classes of arthropods which possess both groups have led to the con- 
clusion that the two sets are complementary to each other, the compound 
eves being adapted to distant sight while the ocelli are myopic, as indi- 
cated by their highly convex lenses. This specialization has been neces- 
sitated by the slight adaptability of the stiff compound lenses to different 
distances. The most primitive ocelli lack lenses entirely and are mere 
pigment spots sensitive only to intensity of light. 
Appendages of cephalothorax. The cephalothorax or prosoma bears 
six pairs of limbs which are homologous with those of Limulus. It is this 
~ close homology and the detailed comparison which is possible between 
