MUSEUM OF COMPARATIVE ZOOLOGY. 109 
described, I believe, the nuclei of this layer, without however correctly 
interpreting them. In his account of the ommatidium in Palemon, 
Grenacher (’79, p. 123) mentions two kinds of bodies in what he takes 
to be the distal ends of the cone cells. Of these, the more distal ones 
(Taf. XI. Fig. 117, ~.) represent, in his opinion, the nuclei of the cone 
cells; the more proximal (Fig. 117, A X’.) he considers as differentiated 
parts of the cone itself. The positions occupied by these bodies in 
Palemon, and by certain bodies which I have observed in Palemonetes 
(Plate IX. Fig. 103), are so similar that I believe the structures in the 
two genera to be homologous. In Palazmonetes the distal bodies lie in the 
cells of the corneal hypodermis (Fig. 103 cl. ern.), and are the nuclei of 
these cells. They represent what Grenacher considered the nuclei of the 
cone cells in Palemon. The proximal bodies in Palemonetes (Fig. 103, 
nl. con.) are unquestionably the nuclei of the cone cells, yet they corre- 
spond to what Grenacher considered the four pieces of the distal segment 
ofthe cone. I therefore believe that what Grenacher has described as the 
nuclei of the cone cells are really the nuclei of the corneal hypodermis, 
and that what he considered distal segments of the cone are the nuclei 
of the cone cells. 
The corneal cuticula in Decapods, in correspondence with the differ- 
entiated condition of the corneal hypodermis, is facetted. The outline 
of the facets is either hexagonal or square. The particular genera in 
which these different kinds of facets occur have already been mentioned 
in dealing with the arrangement of the ommatidia in Decapods. The 
faces of the facets in Decapods are usually very nearly plane, but in 
Palemon according to Grenacher (’79, p. 123), and in Palzemonetes 
(Plate IX. Fig. 103, crn.) according to my own observations, the facets 
are slightly biconvex. In Homarus, as Newton (’73, p. 327) has ob- 
served, and in Astacus according to Carriére (’85, p. 167), the distal 
surface of the facet is plane, the proximal slightly convex. In even 
the most extreme cases, however, the convexity of the facets in Decapods 
is not sufficient to make them very effective as lenses. 
The facets in Decapods are generally bisected by a fine straight line. 
This line, as Patten has suggested, probably represents the plane of 
separation between the two subjacent hypodermal cells. In the square 
facets this line either divides the facet diagonally, as in Homarus 
(Parker, 790", Fig. 2), or transversely, as in Palamonetes (Plate IX. 
Fig. 105). In the hexagonal facets it either bisects opposite sides, as in 
Cancer (Plate X. Fig. 126), or unites opposite angles, as occasionally in 
Galathea (Patten, ’86, p. 644, Plate 31, Fig. 114). Leydig’s (’57, p. 252, 
