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 deeper fibrous one, has resulted in both cases, 

 that these have developed ontogenetically iu 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, whicl» in some of 

 the lower Teleosts (e. g. Siluroids) and iu 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 fusb 

 with the basal plate, but are joined to it by connective-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 witli the ganoin layer cannot be true, for the two 

 develop quite dilferently 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 liomology 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 each represents at present only 

 a highly modified part of the early ancestral prototype. 



