1907.1 



lhe scales op fish. 



771 



Folypterus is of the ganoid type ; and, moreover, that it belongs 

 to the pala3oniscoid variety. For in it can be seen not only the 

 concentric layers of bone below and ganoine above, but also the 

 intermediate hoiizontal network of vascular canals, and the 

 cosmine-like layer supplied by narrow canals from which spring 

 numberless dentinal tubules. iSTo scale of this kind is known 

 outside the Actinopterygii. The evidence is quite clear and 

 definite : the scale is of the true ganoid type, and approaches that 

 of the Palfeoniscoid more closely than any other. Not for a 

 moment is it asserted that Polypterus is a living Palasoniscid ; 

 but it is probably in the neighbourhood of this family that it will 

 eventually find its place in the system of classification. 



The AcANTHODii. — The scales of the Acanthodii are of very 

 uniform structure, usually in the form of a mosaic of very small 

 closely fitting, thick, rhomboidal plates, set in oblique rows like 

 the ganoid scales of a Teleostome. 



They have always been compared to those of the Elasmobranchs, 

 and are generally spoken of as modified denticles, in which the 

 pulp-cavity has become reduced. Rohon (18), Reis (16), and 



Text-fig. 204. 



■ dt 



(From Laukester's ' Treatise on Zoology,' by permission of Messrs. A. & C. Black.)' 



Transverse section of the scale of Acantliodes sp. ; L. Carboniferous, Edinburgh. 

 dt., branching canaliculi ; g., outer shiny layer ; i., inner more opaque layer. 



Fritsch (2) have given descriptions of the scales of various 

 Acanthodians. 



The chief features of the structure of the scale of Acanthodes 

 are shown in the semi-diagrammatic text-figure 204, and in fig. 21 

 (PI. XL VI.), which represents a section taken parallel to the upper 

 surface. It is at once clear that it difi'ers radically from the placoid 

 denticle in its mode of growth, which is by the addition of complete. 



