138 BULLETIN OF THE 
That the general course has been the same in both cases is shown by 
the fact that a broad flat bony plate composed of two layers, an outer 
more homogeneous and a decper fibrous one, has resulted in both cases, 
that these have developed ontogenetically in a very similar manner, and 
have come to overlap one another in similar diagonal rows. This has, 
in both cases, involved the reduction of the spines, which in some of 
the lower,Teleosts (e. g. Siluroids) and in Lepidosteus are present in a 
degenerate condition, but are absent in the greater part of the Teleosts. 
That in some of the lower Teleosts (e. g. Hypostoma) they do not fuse 
with the basal plate, but are joined to it by connéctive-tissue fibres only, 
may be considered an evidence that degeneration has here gone a step 
farther than in Lepidosteus. 
Klaatsch’s idea that the upper layer (Hyalodentin) of the Teleost 
scale is homologous with the ganoin layer cannot be true, for the two 
develop quite differently and their physical and chemical properties are 
very unlike. If, as I believe, Klaatsch did not see the true ganoin 
in Lepidosteus, but mistook for it the outer part of the bony scale below, 
his conclusion in regard to the homology of this layer is in perfect 
accord with that which I have expressed. 
If my conclusions concerning the origin and nature of the outer layer 
(ganoin) of the scales of Lepidosteus is correct, it will follow that no 
very close and direct relationship can exist between these scales and 
teeth. Their only relationship is such as arises from the fact that they 
are both derived from an ancestral condition similar to that found in the 
scales of Selachians. This primitive condition has been modified by 
changes leading in opposite directions. In the mouth, the spines have 
been developed to form the teeth ; on the surface of the body, the basal 
plate has given rise to the scales. Thus cach represents at present only 
a highly modified part of the early ancestral prototype. 
