MUSEUM OF COMI'AUATIVE ZOOLOGY. 73 



colics, have here become fused together to form a continuous inner 

 layer, i. 



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 Typhlogobius retina, ultimately result in a similar 

 thick, solid retinal pigment layer ; this being efi'ected by a still fur- 

 ther fusion of the cross I'ods of pigment now seen. As already pointed 

 out, it is certain, both from Dr. Eigenmann's observations and my own, 

 tliat 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. rtl. ex.), and the layer of nuclei 

 (Fig. 21, nl. ha.) 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 /«/. (Fig. 20) will be noticed in Figure 21. These two layers 

 together seem to correspond to W. Miiller's ('74, pp. GO and 61, Taf. 

 XIII. Figs. 4 and 7) layer of tangential fulcrum cells. This layer is 

 described by this author as being composed in Petromj'zon of " zwei 

 Etagen grosser quadratischer Zellen, zwischen welchen eine Schicht 

 ganz flaclier, in faserartige Auslaufer 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 Percidfe and the Cyprinidse 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 raembrana 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 liere add an observation recently made, which indicates that the time 

 thus passed hidden from tlie 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, 

 wbile at other times hardly any are seen. Whether their absence is due entirely 

 to their having gone into the holes I am not sure ; but however that may be, at such 

 times I have occasionally found them by digging. I am not yet able to say whether 

 tlieir disappearance is in any way correlated with conditions of the weather as 

 regards sunshine. 



