762 MR. E. S. GOODRICH ON [NoV. 12, 



the view that the tvibercles in " Dendrodus " are denticles which 

 have become fused on (19). This is an important point which 

 requires further investigation. 



With regard to the ganoid scale new difficulties arise. The 

 cosmine layer in these is supposed to represent the thoroughly 

 fused denticles. Williamson is not explicit on this point, and 

 does not trace in detail the origin of the scale of Palceonisctis from 

 that of Megalichthys. The denticles, we may suppose, have sunk 

 away from the epidermis, and have become surrounded by 

 concentric layers secreted by the mesodermal pocket in which the 

 scale is now enclosed (text-fig. 200, e & p). But how on such a 

 \iew can we account for the new formation of cosmine in each 

 successive layer ? In the case of the cosmoid scale, the outer 

 layer, we might suggest, remained immediately below the 

 epidermis, new denticles being continually added at the circum- 

 ference. Such a ring of growth may be compared to a dental 

 groove. These denticles might develop in the ordinary way from 

 " germs " at the edge, and become fixed on diu'ing growth (text- 

 fig. 200, d). The thin sheet of transparent substance covering 

 the cosmoid scale would then represent the enamel. But, it 

 may be urged, in the palseoniscoid scale the cosmine would soon 

 become cut oflt" from the epidermis, and it is difficult to see how 

 new " germs " could be produced, and more especially how the 

 newly formed denticles could come to occupy the position of the 

 cosmine in the middle of each lamella at the junction between the 

 ganoine and the bone. This objection is not fatal, for it must be 

 remembered that the inner secreting surface of the pocket probably 

 represents the original upper surface of the dermis as far as the 

 edge of the scale, and therefore might well retain the power of 

 producing new denticle " germs " at the periphery. In the series 

 of diagrams (text-fig. 200, p. 761) I have endeavoured to illustrate 

 this extension of Williamson's theory in accordance with more 

 recently acquired evidence concerning the development of these 

 various structures. 



Of the development of a typical pala^oniscoid scale we know 

 nothing ; but Mckerson (13) has given us a most excellent 

 account of the development of the scale of Lepidostexis, which 

 entirely bears out Williamson's view as to its mode of growth by 

 concentric layers, and I have myself made observations on the 

 development of the scales of Folypterus which lead me to the 

 same conclusion. 



Let us now examine a third difliculty which suggests itself. 

 It is this : if the denticles are already represented in the cosmine, 

 how comes it that the scales of Folypterus, which contain a cosmine 

 layer (p. 770), have denticles set on their outer surface ? This 

 question brings us to the consideration of Hertwig's theory of the 

 origin of scales and dermal bones. 



Agassiz and Williamson described the denticles which are 

 movably articulated to the surface of the dermal bones of 

 certain Siluroid fish {Bypostotna, etc.) ; and Reissner (17) found 



