No. 2.] EYES OF ARTHROPODS. 255 
monograph of North American Devonian crustacea, constituting 
Volume VII. of the Palaeontology of New York. In this place 
are also to be found satisfactory figures of the eye in various 
modes of preservation. 
COMPOSITION OF THE VISUAL NODE. 
The eye is composed of the visual surface, which is normally 
a lunate segment of the surface of a cone, but often in senile 
individuals is inclined to sphericity; this surface is buttressed 
on the glabellar side by a strong palpebral lobe, which is pro- 
duced to and slightly beyond the upper edge of the visual sur- 
face, forming a distinct palpebrum. The lower edge of the visual 
surface is bordered by a ridge, which becomes broader and more 
conspicuous outwardly, and may be called the orbital ridge. 
The lenses as seen from the upper surface are convex, some- 
times being translucent, especially when they have been filled 
with crystalline calcite, or have been slightly separated from the 
matrix; they are circular in outline, although the cavities in 
which they lie (/ensal pits) appear in old individuals to be hex- 
agonal. This appearance is due to the undiminished growth 
beyond maturity of the sclera, which crowds upon and overlaps 
the edges of the lenses on all sides, deepening the lensal pits. 
The general arrangement of the lenses is in alternating vertical 
rows or quincunx. For convenience, however, in the accom- 
panying statements of enumeration, the lenses are regarded as 
arranged in diagonal rows parallel to the lower posterior margin 
of the visual surface, and are numbered from this line consecu- 
tively. It appears probable that this is also the order followed 
by nature in increments to the number of lenses, from the time 
of the formation of the primary row of ommatidia (vide seq.) 
onward to maturity. Under this arrangement the last row in 
enumeration is that ending in the upper anterior angle of the 
visual surface. 
Lhe number of these rows 1s variable, in the majority of cases 
being zzve, in comparatively few individuals of average size and 
richly supplied with lenses, being ¢ez, in extremely rare instances 
eleven, only a single example of average size showing so many; 
in very young individuals the number of rows is but e¢gz. The 
youngest specimens observed show no less than this number, 
and it seems probable from other considerations that in still 
