268 CLARKE. [Vou. II. 
EXPLANATION OF PLATES. 
Fics. 1-7. Schematic representations showing the different modes of preservation 
observed in the visual surface of Phacops. Each figure represents a single lens with 
the lensal or ommatidial cavity, and the adjoining sclera or its equivalent space. 
1. The cornea and sclera normally retained, the lensal and corneal cavities being 
filled with matrix. The lens is represented with its posterior cavity having the rela- 
tive size indicated by the specimens figured elsewhere on the plate, and the anterior 
corneal cavity is characterized by radiating lines which are intended to show the 
difference often apparent in the character of the matrix filling the cornea. 
2. The cornea removed and the sclera retained, the matrix showing the position 
and size of the posterior corneal cavity, but retaining no indication of the anterior 
cavity filling, which is removed with the cornea. 
3. The cornea retained and the sclera removed, the lens standing at the summit 
of a pillar of matrix which represents the ommatidial cavity. 
4. Both cornea and sclera removed, \eaving pillars of matrix with a cup-shaped 
summit, in the bottom of which lies a little ball. The pillar represents the ommati- 
dial cavity; the concave summit, the lower surface of the lens, and the little ball, the 
posterior corneal cavity. 
5. An external film is removed from both cornea and sclera, destroying the for- 
mer and leaving the filling of the anterior corneal cavity standing out prominently 
with almost the full size of the lens. In the single instance observed of an eye in 
this condition, the posterior corneal cavity is empty in all the ommatidia that were 
opened. The reason for this is not well understood. The sclera has been silicified, 
and subsequently decomposed, so that it is indistinguishable from the matrix. 
6. The outer and inner wails of the visual area have been replaced by a film of 
silica, and the rest of the calcareous matter subsequently removed, leaving both cornea 
and sclera preserved as a mere shell. 
7. The same condition of preservation, modified by the adherence of the cornea 
and outer wall of the sclera to the matrix outside the eye, leaving the walls of the 
ommatidial cavities adhering to the internal matrix as a series of short tubes. This 
mode of fossilization has not been observed in Phacofs rana, but is not uncommon in 
specimens of Phacops cristata, var. pipa, from the decomposed Upper Helderberg 
phtanite. 
Fics. 8-22. Schematic representations of the lenses of the visual surface. The 
curved surface is projected upon a plane, and the relative size and position of the 
lenses is retained. Whether the representation is from a right or left eye, the 
lower posterior margin is at the right of the figure, the diagonal rows being enumer- 
ated from this side, obliquely downward from right to left. All the figures are drawn 
to the same scale. 
8. The visual surface of an extremely young individual measuring 6 mm. across the 
base of the cephalon; composed of 31 lenses in 8 rows, nearly all the terminal lenses 
being immature. The older lenses show a tendency to arrangement along a single 
or double transverse row, parallel to the margins of the visual surface. 
g. An older eye, bearing 35 lenses in 8 rows, belonging to a young Phacops, hav- 
ing a cephalic width of 7 mm. At the upper extremities of the rows the lenses are 
all full-grown, and all immature at the lower extremities. 
Io. An eye composed of 40 lenses in 8 rows, and belonging to a young individ- 
ual with a cephalic width of 9 mm. 
