MUSEUM OF COMPARATIVE ZOOLOGY. 109 



described, I believe, the nuclei of this layer, without however correctly 

 interpreting them. In his account of the omniatidium in Palaimon, 

 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. XL Fig. 117, n.) represent, in his opinion, the nuclei of the cone 

 cells; the more proximal (Fig. 117, A'^'.) he considers as diflferentiated 

 parts of the cone itself. The positions occupied by these bodies in 

 Paltemon, and by certain bodies which I have observed in Palajmonetes 

 (Plate IX. Fig. 103), are so similar that I believe the structures in the 

 two genera to be homologous. In Palsemonetes the distal bodies lie in the 

 cells of the corneal hypodermis (Fig. 103 cl. crn.), and are the nuclei of 

 these cells. They represent what Grenacher considered the nuclei of the 

 cone cells in Paltemon. The proximal bodies in Palsemonetes (Fig. 103, 

 7iL con.) are unquestionably the nuclei of the cone cells, yet they corre- 

 spond to what Grenacher considered the four pieces of the distal segment 

 of the 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 

 fiices of the facets in Decapods are usually very nearly plane, but in 

 Palsemon according to Grenacher ('79, p. 123), and in Palaemonetes 

 (Plate IX. Fig. 103, cr«.) 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 Carriere C85, 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 fixcets 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, '90", Fig. 2), or ti'ansversely, as in Palaemonetes (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, 



