No. 2.] EYES OF ARTHROPODS. 269 
11. An eye with 42 lenses in 8 rows, belonging to a large individual, having a 
width across the cephalon of 34 mm. The lenses are nearly all mature, but are abnor- 
mal in their arrangement, showing a failure to develop properly at the upper extremi- 
ties of the rows after the third. 
12. An eye of Phacops cristata, var. pipa, composed of 50 lenses, and showing 
an abnormal arrangement, a single mature lens in the last row being situated high up 
in the base of the palpebrum. Had it developed in the vacant space in the last row 
but one, the arrangement of the lenses would have been normal for this variety. 
13. An eye of the same variety bearing 52 lenses in 8 rows. Eight appears to be 
the normal number of rows in the mature eye of this form, the anterior vertical row, 
and the last diagonal row, consisting of three lenses. In the other diagonal rows 
there is apparently much greater variation than in the eye of Phacofs rana. 
14. Aneye of Phacops rana composed of 57 lenses in 9 rows, the head having a 
basal width of 52mm. This is the eye of a senile individual, and all the lenses are of 
mature size with the exception of those on the palpebral margin. The number of 
lenses in the anterior vertical row is three instead of four, as in the normal adult. 
15. Another senile eye, with 60 lenses in 9 rows, belonging to a very large speci- 
men, with a cephalic width of 7omm. Here again the lenses are all of full size with 
the exception of those on the upper margin of the visual surface; the lenses of the 
anterior vertical row are also three in number. 
16. An eye composed of 60 lenses in 9 rows belonging to an individual slightly 
above normal adult size, having a cephalic width of 28mm. In the first row one 
interval has been skipped in the addition of the last lens. 
17. An eye of normal size and development, bearing 70 lenses in 10 rows, the 
cephalon having a width of 24mm. The first row consists of a single full-grown lens 
at a considerable distance from the posterior extremity. 
18. An eye from an individual of the same size, having 70 lenses in 9 rows. 
19. An eye composed of 71 lenses in 10 rows, the cephalon to which it belongs 
having a width of 27 mm. It essentially differs from the preceding only in the pres- 
ence of the single lens constituting the first row. 
20. An eye with 75 lenses in 10 rows, from an individual having a cephalon 28 mm. 
in width. 
21. An eye composed of 88 lenses in II rows, from an individual measuring 
17 mm. in cephalic width. This is the greatest number of lenses and rows of lenses 
noticed in this species. 
22. An average eye of Dalmanites Boothi, var. Calliteles, Green, consisting of 206 
lenses in 29 rows. 
Fic. 23. A left eye of Phacops rana, enlarged to 3 diameters, showing the arrange- 
ment of the lenses, the smaller size of several of the terminal lenses, and the tuber- 
cles on the integument of the palpebrum and orbital ridge. The lensar cavities are 
represented as much too sharply hexagonal; they should be more rounded and exca- 
vate. The figure is copied from the Palzontology of New York, Vol. VIL., Pl. 8, Fig. 6. 
Fic. 24. A portion of the visual surface of a very old eye, taken from an individ- 
ual measuring 70 mm. across the base of the cephalon; showing the thick and deeply 
excavate sclera, the full-grown lenses along the lower margin, and the small lenses at 
the upper margin of the area. Enlarged to 3 diameters. 
Fic. 25. A natural cast of the internal surface of a portion of the visual area in 
Phacops cristata, var. ipa, enlarged to 6 diameters. The specimen shows very beau- 
tifully the cup-shaped casts of the ommatidial cavities, each with a little ball at its cen- 
tre representing the posterior corneal cavity. At the upper extremities of the last 
