362 On the Age of the Sandstones of the Newark Group. 



of these fishes in Europe. Based on this designation, Sir Philip 

 Egerton proposed his new genus Dictyopyge for the C. macrurus 

 of the Virginia rocks. 



In regard to the other fishes of New England and New Jer- 

 sey, Mr. J. H. Kedfield had reluctantly followed the work of Prof. 

 Agassiz in assigning them to the genus Palceoniscus, although 

 this eminent naturalist had then only seen two imperfect speci- 

 mens ; but Mr. R. then alluded to their structural affinity with 

 the liassic fishes, as we have seen in his conclusion already 

 quoted, and impliedly in the descriptive portion of his paper. 

 It is well seen, also, in his figure of the P. latus, 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- 

 culiar structure of their fins warrant their being placed in a sepa- 

 rate genus. Sir Philip Egerton recognizes the division, as did 

 Prof. Agassiz in 1846, and Sir Philip proposes for the new genus 

 the name Ischypterus. 



The question to which of the divisions of Agassiz the Catop- 

 terus of Connecticut and this fish of Virginia belong, is simply 

 one of degree. Even if we were to admit a slight difference in 

 this case, it could hardly imply the wide separation which has 

 been claimed. Such a marked division, founded on the struc- 

 ture of the tail, cannot depend on the use of a term, but must 

 be decided by the fishes themselves. 



In regard to this point of distinction, may I not quote the 

 matured views of Sir Philip Egerton, so well expressed in the 

 Journal of the Geological Society, 1854, p. 368: — "Although 

 this character, derived from the organization of the caudal fin, 

 is one of great value and significance in the determination of 

 various genera of fossil fishes, it is nevertheless necessary, in 

 drawing general conclusions, to be careful not to assign to it 

 more importance than it is strictly entitled to ; for we find, by 

 the comparison of several genera, that it is not one of those well 

 defined trenchant characters which can be affirmed to exist or 

 not, as the case may be, but that it is variable in amount, pass- 

 ing from extreme heterocercy to absolute homocercy by a sliding- 

 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 palaeozoic and the mesozoic strata ; and perhaps in lesser de- 

 gree, at the close of the triassic period. 



In all our Gatopteri the scales of the caudal base terminate 

 near the middle rays of the upper lobe, "and not on the upper 

 margin, as in a true heterocerque tail."* Good figures by Din- 



* See Egerton as last quoted p. 3*70. 



