772 MR. E. S. GOODRICH ON [Nov. 12, 



concentric layers, just as in a ganoid scale. Neither in this 

 nor in any other species that I have examined, from Devonian, 

 Carboniferous or Pei-mian rocks, can I find the slightest trace of 

 the small pulp-cavity the presence of which is asserted by Rohon, 

 but denied by Reis. Nor does the structvire of the scale seem to 

 me to afford any evidence whatever of such a cavity having been 

 present. Reis and others have already described dentinal tubules 

 passing inwards from the periphery. These run in towards the 

 centre, at right angles to the lines of growth, from all the cir- 

 cumference except the top ; and are specially numerous about 

 half way up the scale, where a slight constriction separates the 

 upper from the lower region. Only a few tubules reach the 

 middle, most of them stop short at varying distances from the edge. 



The outer or upper region of the scale appears more dense than 

 the lower ; it is not pierced by tubules from above, but some of 

 them penetrate far into the superficial laminaa from the sides 

 No bone-cells are present. 



The Acanthodian scale, with its concentric laminfe and upper 

 ganoine-like layers, bears a striking resemblance to the ganoid scale 

 of the Teleostome. Moreover, the distribution of the branching 

 dentinal tubules is quite like that of the similar tubules of the lepid- 

 osteoid type of scale, excepting for their more extensive spreading 

 on to the upper surface. There are no vascular canals, a fact 

 which may be correlated with the small size of the scale. 



Whatever may be the origin of the Acanthodian scale, there 

 can be no doubt that it is built on a quite different plan from 

 that of the placoid, and that it approaches most nearly to the 

 lepidosteoid tyP®- Fritsch (2) figures scales of Traquairia and 

 Protacanthodes with a tooth-like process on the hinder edge ; 

 unfortunately he gives no information as to the histological 

 structure of these scales, so that it cannot be determined whether 

 the process represents a pointed extremity or a denticle 

 fused on. 



At present we know too little concerning the scales of the 

 Acanthodii, to draw any conclusion from their structure as to 

 the phylogeny of the group. But these scales should no longer 

 be vaguely called modified denticles, and used as evidence of 

 •afiinity with the Elasmobranchs. 



List of References. 



1. Agassiz, L. — Recherches sur les Poissons Fossiles. Neuchatel, 



1833-44. 



2. Fritsch, A. — Fauna der Gaskohle. Prague, 1879-94. 



3. Goodrich, E. S. — " On the Dermal Fin-Rays of Fishes." 



Quart. Jour. Micros. Sci. vol. xlvii., 1904. •» 



4. Goodrich, E. S. — " On the Systematic Position of Polypterus" 



Brit. Assoc. Report, 1907. 

 4a. Goodrich, E. S. — "On the Pelvic Girdle and Fin of Eu- 

 sthenopteron.^' Quart. Jour. Micros. Sci. vol. xlv., 1901. 



