No. 2.] EVES OF ARTHROPODS. 263 
NoTE. — No satisfactory evidence of crystalline cones within the ommatidial cav- 
ities has been ascertained, and it is not surprising that these bodies, which undoubtedly 
existed, were removed with the soft parts of the visual organ. With respect to this 
feature the sections of the eye of Phacopfs given by Barrande (Systéme Silurien du 
Centre de la Bohéme, Vol. I., Pl. 3, Figs. 15 and 16), and reproduced by Zittel 
(Handbuch der Palewontologie, 1885), are misleading. The fillings of the ommati- 
dial cavities are so shaded as “to indicate prisms” (compound) “ corresponding ~ 
with each lens,” and extending very far inward without diminution in width. Such 
structure finds no correspondence in the eye of the living Arthropod, and is prob- 
ably to a large degree schematic and imaginary. The structure of the lens, as we have 
found it, is also essentially different from that represented by Barrande. 
MopES OF PRESERVATION OF THE VISUAL SURFACE. 
a) The cornea and sclera are normally preserved. (Fig. 1.) 
This is the usual mode of preservation in the limestones 
where the original substance of the test has been preserved in 
calcic carbonate, though leaving so considerable a portion of the 
organic matter as to give a black and lustrous surface. Such 
specimens retain the minute structure of the test most perfectly 
and are most satisfactory for sectioning. 
b) The cornea ts removed and the sclera retained. (Fig. 2.) 
This is a rare mode of occurrence noticed only in specimens 
from the shales. 
c) The sclera 1s removed and the cornea retained. (Fig. 3.) 
In these examples, which occur in the shales and weathered 
limestones, the corneal lenses stand supported on the summits 
of pillars of matrix. It is not an uncommon mode of occurrence. 
ad) Both cornea and sclera ave removed (Fig. 4), leaving pillars 
of the matrix with cup-shaped upper surfaces, each bearing a 
little ball at the centre. This condition is often observed in the 
decomposed limestones and phtanites. 
e) An external film ts removed from the entire visual area, 
destroying the cornea (Fig. 5). A single example has been 
observed in which the entire test was apparently originally pre- 
served in calcic carbonate. Subsequently this was removed 
from within and gradually replaced by silica, with the exception 
of the thin outer film, which afterward was entirely removed, 
leaving the space it occupied vacant. 
J) Silica deposited as a thin film upon, or replacing a thin film 
of the external and internal surfaces of the test, and all the rest of 
the substance of the test and the matrix removed (Figs. 6 and 7). 
In this condition the visual surface is a mere shell appearing as 
