266 CLARKE. [VoL. II. 
CONCLUSION. 
The study of the eye in Phacops rana as here presented al- 
lows the statement of the following points : — 
1) The schizochroal eyes of the Trilobites are aggregated 
and not properly compound eyes. ‘The visual organs of Harpes 
may prove to be of similar character. 
2) The scleral portion of the visual surface is of the same 
structure as the test, and is a direct continuation of it. 
3) No evidence appears of any continuous corneal layer cov- 
ering the entire surface. 
4) The corneal lenses are wholly discrete from the epidermis, 
but are of epidermal origin. In the addition of new lenses to 
the visual surface, they appear to arise from a thinning of both 
surfaces of the integument. 
5) The corneal lenses were hollow or filled with some matter 
not homogeneous with the cornea itself. 
6) The corneal lenses, and therefore the ommatidia, are 
added to the visual surface with advancing age until the mature 
growth of the individual is attained; thereafter they diminish 
in number with increasing senility. 
7) The addition of corneal lenses occurs regularly at the ex- 
tremities of the diagonal rows. 
8) No evidence is preserved of crystalline cones in the om- 
matidial cavities, though they may have been removed in the 
decomposition of the soft parts of the eye. 
In conclusion I wish to call attention to the primitive struc- 
ture of the eye exhibited in a Devonian subtype which is provis- 
ionally referred to the order of the Phyllocarida. Dr. J. S. 
Kingsley in the final paragraph of a valuable paper on the 
“Development of the compound eye in Crangon”’ (/ournal of 
Morphology, Vol. 1., p. 63), has written: “The observations as 
yet recorded are not sufficient to throw any great light on the 
phyllogeny of the Arthropod eye; still one or two points may be 
feature of difference from the schizochroal eyes. My own study of the holochroal 
eyes has not been as careful as I hope to have the opportunity of making it, but it 
may be observed that sections of the eye in Proé‘us Rowi seem to indicate a very 
tenuous interlensar sclera; moreover, the immature Calymene senaria referred to in 
a previous footnote shows evidence of such interlensar integument. 
