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



found in our rocks differ somewhat from each other, also, in the 

 degree of heterocercal structure which they present, those spe- 

 cies which, following Prof. Agassiz in P. Julius, I have allotted 

 to the genus Palceoniscus, having the heterocercal structure more 

 decided. But even in these, the tail has a different aspect from 

 the Palceomsci of Europe. In the latter, the upper lobe of the 

 tail seems hardly to partake of the character of a fin, and the 

 lower lobe appears to be only a fin-like appendage of the upper, 

 like a second anal fin, while the scales and no doubt the vertebrae 

 extend to the extreme point of the upper lobe." 



" The other genus, the Catopterus of our rocks, exhibits the 

 heterocercal structure in a still more modified degree. So nearly 

 does it approach in this respect some genera classed as homocer- 

 cal fishes, such as Semionotus and Pholidophorus, that in an early 

 memoir published in the Annals of the Lyceum of Natural His- 

 tory, vol. iv, I was led to rank it in that division, subject to a 

 qualifying note. Its relations are however, rather to the hetero- 

 cercal fishes, or perhaps to an intermediate group." 



"This point is an important one in its bearing upon geo- 

 logical questions, for it is now well ascertained that the true 

 heterocercal tail [in the lepidoids] is peculiar to the palaeozoic, 

 and lower mesozoic rocks, no fish of that character having been 

 found higher in the series than the triassic rocks, while the true 

 [strict] homocercal tail does not occur below the lias. When 

 therefore we find in the fishes of our sandstone rocks, a struc- 

 ture which seems to be intermediate between the true homocer- 

 cal and the heterocercal divisions of Agassiz, the conclusion 

 seems irresistible that the including rock cannot be older than the 

 triassic, while it must be placed at least as low in the series as the 

 lias or oolite." Report, pp. 5-6. 



"■ — ■ Only four species of the genus Catopterus are yet known ; 

 three of which are found in the red sandstone of New England 

 and New Jersey and the fourth in the coal rocks of Eastern Vir- 

 ginia."* Report, p. 7. 



His descriptions of these four species of Catopterus are found 

 in the report, and were then prior to any known notice or de- 

 scription of these fishes, other than our own, and together with 

 the descriptions of the more numerous species of the genus 

 Ischypterws, are yet withheld from publication, on account of the 

 contemplated arrangements for completing a monograph of the 

 fishes of this formation in the United States. 



I have thus shown the examinations and conclusions of Mr. J. 

 H. Redfleld on these fishes, as first published in 1837, and as 

 found in his report to the American Association in 1845. In 

 the first of these he points out the age of the containing rocks, 



* Others have since been obtained. 



