418 The American Naturalist. [May, 

FI a een althausii, }. mpe the order Docopteri, an actinoptery- 
gian fish fi rom the Trias, with well developed baseosts of the median fins, and axonosts 
apparently Sona with the neural spines; from Zittel. 
4. ON THE CAUDAL FIN AND ITS SUPPORTS. 
Professor Louis Agassiz first called attention to the diversities 
in the structures of the caudal fins of fishes, and their relation to 
the general history of the class. He showed that the tails of 
modern fishes are constructed on two different patterns, which he 
called the homocercal and the heterocercal. In the former, the 
radial portion of the fin is in two unequal lobes, the superior being” 
the more produced distally, and the inferior occupying a position 
on its inferior aspect, and having a much less degree of posterior 
prolongation. In the latter, the inferior lobe is of equal length 
and posterior prolongation with the superior. He showed that 
the homocercal type is characteristic of all Paleozoic and most of 
the Mesozoic fishes known to him, while the heterocercal type 
predominates in the Cenozoic and existing fishes. He also 
showed by a study of the embryology of the trout and other 
forms, that in their earlier stages the heterocercal fishes are homo- 
cercal. These results he regarded as important, and, in fact, they 


