362 On the Age of the Sandstones of the Newark Group. 
of the Virginia rocks. 
In regard to the other fishes of New England and New Jer- 
ab and impliedly in the descriptive portion of his paper. 
+ is well seen, also, in his figure of the P. Jatus, attached to his 
paper in the Annals. In my own notices of 1841, referred to 
above, I suggested that their less heterocercal forms, and the pe- 
ppterus. 
The question to which of the divisions of Agassiz the Catop- 
In regard to this point of distinction, sia T not quote the 
; : 
3 
not, as the case may be, but that it is variable in amount, pas* 
ing from extreme heterocercy to absolute homocercy by a sli ing- 
scale so gradual, that it is (at all events in fossil examples) most 
difficult to define a positive line of demarcation between the two 
forms.” 
_ As the terms have hitherto been used, such line of demarca- 
tion, if it exist, appears best indicated at the division between 
the palzeozoic and the mesozoic strata; and perhaps in lesser de- 
gree, at the close of the triassic period. 
In all our Chtopteri the scales of the caudal base terminate 
near the middle rays of the upper lobe, “and not on the mpen 
margin, as in a true heterocerque tail.”* Good figures by Vr 
* See Egerton as last quoted p. 370. 
