No. 2.] EYES OF ARTHROPODS. 259 
hollow. It appears, also, in sections that zz the mature lens the 
cornea is discrete from the sclera, lying in juxtaposition with 
and held in place by it, but in nowise continuous with it. This 
fact is also frequentiy apparent in specimens from which the 
sclera has been removed by solution, leaving the corneal lenses 
standing on little pillars of the matrix, which has filled the 
ommatidial cavities. Other specimens just as frequently show 
the converse, the lenses being removed, while the sclera is 
retained. There is evidence which I deem worthy of considera- 
tion, that these corneal lenses, during the life of the animal, 
were not empty, but filled possibly with some viscid or spissate 
humor. This evidence is of the following character :— 
The cornea itself is thin, and of even calibre throughout its 
extent. The casts of the corneal cavities, such as are shown in 
Figs. 25 and 26, as little balls lying in cups, are not of sufficient 
size to have occupied all the space within the cornea. A speci- 
men of Phacops of a species closely allied to vana (Ph. cristata 
var. pzpa Hall), from the decomposed phtanite of the Cornifer- 
ous limestone, seems to demonstrate this fact. This fossil 
was evidently originally preserved in calcic carbonate, which 
not only replaced the entire crust, but filled the ommatidial 
cavities, and the posterior cavity of the corneze as well. This 
calcic carbonate was subsequently removed from within, and 
its place partially taken by silica, and when exposed to more 
rapid decomposing agencies, the remainder of the calcic car- 
bonate was removed, leaving the fossil so preserved that 
the cornea and a thin film over the entire external surface 
of the sclera has been taken away, the remainder of the test 
being replaced by silica. The external surface of each lensal cast 
remains convex, but on carefully removing a little of the decom- 
posed rock from beneath the position of the cornea a vacant 
space appears, which corresponds to the corneal cavity as repre- 
sented by the ball-in-cup casts. To elucidate this point, see 
Plate XXI., Fig. 5, and explanation thereof. 
Again, certain well-preserved sections from the limestone 
show a distinct difference in the character of the matrix fill- 
ing the outer and inner cavities of the cornea, that in the outer 
being of lighter color, and more translucent (? subcrystalline), 
while that in the inner is the opaque mud of the sediment. 
More evidence upon this point is very desirable, but enough 
