1880. | The Structure of the Eye of Trilobites. 507 
trilobite (Asaphus gigas, Fig. 2), we see that the eye is raised 
upon a tubercle-like elevation of the carapace; the integument 
(int) is about as thick as that of Limulus, and it contains similar 
pore-canals (fc); the eye itself, or cornea, occupies a rather 
small area; its exterior surface, instead of being smooth as in 
Limulus, is tuberculated, or divided up into minute convex areas; 
these convexities are the external surfaces of the corneal lenses, 
which extend through the cornea, so that its surface is rough 
instead of smooth as in Limulus; c/ indicates one of the corneal 
lenses which are arranged side by side; they are of slightly dif- 
ferent lengths and thicknesses, and the rather blunt free ends pro- 
ject into the cavity of the eye, which in the fossil is filled with a 
translucent calcite. 
It is quite apparent that we have here the closest possible 
homology between the hard parts of the eye of Limulus and the 
Asaphus. Another point of very considerable interest is a toler- 
ably distinct dark line (r¢) which seems to run across from one 
lens to another, and which may possibly represent the external 
limits of the retina or pigment mass in which the ends of the 
lenses were probably immersed; should this be found to be the 
indications of the outer edge of 
the retina, it would be a most 
interesting fact in favor of our 
view of the identity between the 
eyes of the two types of Paleo- 
carida under consideration. 
Another section sent us by Mr. 
Walcott is represented by Fig. 3; 
itis from Asaphus gigas, but rep- 
resents a less elevated and broader Fic, 3.—Cornea of Asaphus. 
part of the eye than that seen in Fig. 3; the section does not so 
well exhibit the free ends of the corneal lenses. Fig. 3 @ repre- 
Sents a transverse view of the eye of Asaphus gigas, 
showing the hexagonal form of the facets, and their 
quincunx arrangement. 
This hexagonal appearance of the corneal lenses is still Fic. 3a. 
retained in natural vertical sections of eyes of the same genus; 
where with a good Tolles lens the sides of the cones are seen to 
be angular. Fig. 4 represents a few such cones, I do not under- 
Stand to what this hexagonal appearance is due; for both in 
