MUSEUM OF COMPARATIVE ZOOLOGY. is 
cones, have here become fused together to form a continuous inner 
layer, 7. 
So far as I have been able to determine, this condition is peculiar to 
Clevelandia, at least to the extent in which it is here seen. ‘The inter- 
esting question now arises whether we have here the beginning of a 
process that would, under conditions that have brought about the 
changes seen in the yphlogobius retina, ultimately result in a similar 
thick, solid retinal pigment layer; this being effected by a still fur- 
ther fusion of the cross rods of pigment now seen. As already pointed 
out, it is certain, both from Dr. EKigenmann’s observations and my own, 
that Clevelandia spends some time at least in holes in the ground.’ 
The only doubt existing concerning the identification of the layers of 
the retina is with reference to what I have called the outer reticular 
layer (Plate III. Figs. 18 and 21, st. rél. ex.), and the layer of nuclei 
(Fig. 21, nl. ba.) that has been designated by the non-committal term 
of “basal nuclei,” basal, i. e., with reference to the outer nuclear 
layer. On comparing Figure 21 with Figure 20, the section of a Cleve- 
landia retina, there will be little doubt of the correspondence of layer 
nl. ba. in the two cases; but at the same time the entire absence of 
layer ful. (Fig. 20) will be noticed in Figure 21. These two layers 
together seem to correspond to W. Miiller’s (’74, pp. 60 and 61, Taf. 
XIII. Figs. 4 and 7) layer of tangential fulcrum cells. This layer is 
described by this author as being composed in Petromyzon of “ zwei 
Etagen grosser quadratischer Zellen, zwischen welchen eine Schicht 
ganz flacher, in faserartige Ausliufer sich fortsetzender Zellen gelagert 
ist.” The layer is said, in the same connection, to be subject to much 
modification in the different families of fishes, in which alone it is well 
developed ; but the Percidze and the Cyprinide are mentioned as teleos- 
tean groups in which the layer with both its “ Etagen” is present. Ac- 
cording to this interpretation the external granular layer of M. Schultze, 
called in this paper the external reticular layer and by Krause (’76) 
the membrana fenestrata, is not present in either Clevelandia or Typhlo- 
gobius; and it is instructive to note that Krause does not find this 
1 I may here add an observation recently made, which indicates that the time 
thus passed hidden from the light is not inconsiderable. On some of my visits to 
the beach at West Berkeley I have found the fish very numerous in the tide-pools, 
while at other times hardly any are seen. Whether their absence is due entirely 
to their having gone into the holes Iam not sure; but however that may be, at such 
times I have occasionally found them by digging. Iam not yet able to say whether 
their disappearance is in any way correlated with conditions of the weather as 
regards sunshine. . 
