FISHES OF LAKE SUPERIOR. 251 



round the tail, towards the abdominal region. In the sharks we 

 have distinct vertical fins, as they generally grow out of the continu- 

 ous, embryonic odd fin ; whilst in skates these fins disappear almost 

 entirely, or are considerably reduced. That animals in their embry- 

 onic condition are neither so elongated as many of cylindrical form in 

 their full-grown state, nor so short as some others, is ascertained by 

 the embryology of snakes and toads. Thus, all the great external 

 differences which exist between skates and sharks on one side, and 

 Petromyzon on the other, do not show that these animals do not 

 belong to the same natural group, as we have even among the full- 

 grown ones, what we may call transitions between the extreme forms ; 

 for instance, sharks with more elongated body than others, with more 

 extensive vertical fins, even with two dorsals and some without ven- 

 trals. Again, the remarkable form of skates arises solely from an 

 extraordinary development of the pectorals ; they are nevertheless 

 closely alHed to sharks, notwithstanding the striking difference in the 

 position of the gill-openings. 



As for the anatomical differences which exist among these fishes, 

 and upon which so much stress is placed as to make the want of a 

 heart, in Amphioxus, the foundation for a peculiar class to include that 

 single fish, let us not forget, that there is an epoch in embryonic life, 

 when no vertebrated animal has yet a heart ; when the vertebral 

 column is a mere soft continuous cord ; when the brain is scarcely 

 subdivided into lobes ; when the head, as such, is not yet distinct 

 from the trunk ; when the mouth is a mere circular opening at the 

 anterior extremity of the body ; when the gills are simple fissures on 

 the sides of the head, or at what is to be a head, without branchio- 

 stegal rays or operculum, or protecting covering of any kind. 



Whoever is famiUar with the anatomy of fishes must perceive, after 

 these remarks, that the peculiarities which characterize Petromy- 

 zon, have a bearing upon the embryonic condition of their structure 

 even in their full-grown state, and do not by any means mark a dif- 

 ference between them and the sharks and skates, any more than 

 between them and any other family of fishes. On the contrary, 

 should it be possible, after these statements, to show that there are 

 important characters, common to Petromyzon, sharks and skates, 

 notwithstanding their extreme external differences, it should be 



