DESCRIPTION OF THE FOSSIL REMAINS. 57 



such difflTent organs, to be kindred to the other! Wide asunder in geo- 

 graphical area as these interesting relics were detected, they are in their 

 geological horizon and position still further apart — the one occupying the 

 lowest and the other the upper series of the Old Eed formation. My unprac- 

 tised eye, when I first saw them and compared them side by side, could 

 neither guess nor decipher anything of their family relationship. 



But it was chiefly upon the characters exhibited by the Dura Den fossil 

 that the genus was established. The resemblance to the holoptychius was 

 first observed, and the scales were supposed to be identical. But several 

 points of distinction were very marked, both in these and in other appendages 

 of the fish. Thus the scales in platygnathus are rounded, broader than long, 

 especially towards the tail. They are likewise ornamented by horizontal 

 well-marked lines, often interrupted, and although placed in rows they do 

 not form continuous crests. The lines, besides, are not so well defined nor 

 so strongly relieved as in the holoptychius. The tail, again, of platygnathus 

 is very long and contracts very gradually, which characteristics are very dif- 

 ferent in the other genus. The fins are also much larger, the dorsals, of 

 which there are two, being at least twice the height of the tail, and composed 

 of elongated close flexible rays. The posterior extremity of this organ is 

 slightly raised, and on the inferior edge there may be observed a number of 

 short rays, which probably composed the under lobe of the fin. Under the 

 belly of the fish a narrow and very long fin can be distinguished, which 

 seems to be the anal ; it ranges along the tail, and appears to have been as 

 wide as the dorsal when spread out. 



The great peculiarity, then, of this genus of fishes, is the enormous deve- 

 lopment of the fins. They are all, caudal and dorsal, of the largest dimen- 

 sions, and heterocercal, and the vertebral column extending to the extremity 

 of the finny appendages, it is justly inferred that they must have been 

 powerful swimmers. The same organs indicate their voracity and great 

 developnient of jawbone and teeth. Hence the connexion presumed to exist 

 between the Orcadean and the Dura Den fossils ; and, found so wide apart 

 as in the inferior and the upper members of the Old Eed, the interest in their 

 discovery is all the more important and heightened by the circumstance of 

 their being solitary examples of their respective species, in the long-separated 

 periods of the geologic relations. 



1. Holoptychius Anderso7ii.- — Agassiz. Pl. I. VII. VIII. 



2. Holoptychius Flemitigii. — Agassiz, 



The yellow sandstone of Dura Den may pre-eminently be distinguished as 

 the habitat of this race of fishes, since so many hundreds have been disinterred 

 from a single bed of the deposit, and nearly all of the same species, the Holo- 

 ptychius Andersoni. Our description, therefore, of this fertile family will 

 admit of being as particular and minute as we have access to the writings of 

 its most celebrated historians. 



The Scales. — The characters of the scales are remarkably striking in all 



