48 H. W. MARETT TIMS. 
between Selachians and Teleosteans of which Nickerson 
speaks does not exist. 
It is possible that the difference between the observations 
of Klaatsch and those just related may be more apparent 
than real. If the vertical extent of the scale-cavity were 
reduced, which quite conceivably might be the case in corre- 
lation with the varying size of the scale, then the roof and 
floor of the cavity would be approximated and the scale-anlage 
lying on the surface of the latter would give the appearance 
as if arising in the midst of the mass of cells. Such, I 
believe, from a comparison of the figures, is a very possible 
explanation of the differences. 
Following upon this stage is the laying down of the cal- 
careous material. J am not ina position to say whether this 
appears upon the surface of the homogeneous layer as a kind of 
secretion, or whether the homogeneous layer itself becomes 
impregnated, but I incline to the latter view. One point is, 
however, certain, namely, that the calcareous material does 
not form an uninterrupted layer, but is in the form of minute 
isolated platelets, the external surface of each of which bears 
a minute pointed and backwardly directed spine; in other 
words, each is a minute microscopic placoid scale: 
this is shown in Fig. 4. These are placed upon a basal 
membrane which stains with borax carmine, and which can 
be seen extending upwards between the contiguous ends of 
adjacent denticles. 
This condition has been entirely overlooked by previous 
observers, the reason being not far to seek. The skin has 
been treated with an acid, either for the purpose of 
decalcifying, or following the routine method of placing in 
acid alcohol after staining with borax carmine. I have been 
careful to avoid all trace of acid in the preparation of the 
specimens, and thus prevent any decalcification. Naturally, 
the sections are somewhat liable to be torn, and are difficult 
to cut, but the results are sufficiently satisfactory to leave no 
doubt as to the existence and mutual relationships of these 
minute placoid scales. In order to test this explanation, I 
