110 BULLETIN OF THE 
Fig. 134) figure of Astacus, in which each facet is subdivided by two 
diagonal lines into four areas, and Newton’s (’73, p. 327) statement 
that the same condition occurs in Homarus, are probably incorrect. 
The cones in Decapods are composed of four segments. This number 
-was first observed by Will (’40, p. 13) in Paleemon, and has since been 
recorded in many other genera. So far as I am aware, there are no 
Decapods in which the number of segments is not four. As Claparede 
(60, p. 194) first pointed out in Galathea and Pagurus, each segment 
contains a nucleus and represents a single cell. Although the signifi- 
cance of these nuclei was without doubt first fully appreciated by 
Claparede, it is probable that they were previously seen by Leydig 
(55, Taf. XVII. Fig. 31) in the crayfish. 
As a rule, the distal termination of the cone cells is on the proximal 
side of the corneal hypodermis. In the lobster, however, and in Pale- 
monetes (Plate IX. Fig. 104), the pointed ends of these cells pass 
between the two cells of the corneal hypodermis, and probably come 
in contact with the corneal cuticula near the middle of a facet. 
- It is difficult to determine with accuracy the proximal termination of 
the cone cells. They can be easily traced to a region immediately distal 
to the distal end of the rhabdome. In this region, as Schultze (’68, 
Taf. I. Figs. 9 and 11) has clearly demonstrated in Astacus, the fibrous 
ends of the four cone cells separate, and pass partially around the rhab- 
dome. In Homarus, these fibres extend proximally, and finally ter- 
minate at the basement membrane. A similar method of termination 
also occurs in Palinurus. In the other genera which I have studied, the 
fibres, although visible near the distal end of the rhabdome, are lost in 
the adjacent tissue, and I do not know whether they terminate in this 
tissue without special attachment, or whether they make their way as 
excessively fine fibres to the basement membrane. The separation of 
the fibrous ends of the cone cells, near the distal end of the rhabdome, 
has been observed by Steinlin (’66, p. 93) in Palemon, and by Schultze 
(’67 and ’68) in several other Decapods. The statement made by many 
of the older investigators, and recently reaffirmed by Patten, that the 
cone and rhabdome are parts of one continuous structure, is without 
doubt incorrect. 
The resolution of the retenuda into its cellular constituents was first 
attempted in Decapods by Leydig (’55, p. 408), according to whom the 
retinula of Herbstia contains four cellular bodies, the nuclei of which 
can be distinguished in the distal part of the structure. A somewhat 
similar condition was described by Newton (’73, p. 333) for Homarus ; 
