244 On Lamprey Eels — {Petromyzonlidce) 



we introduce another principle, by which we can weigh the 

 real value of those remarkable differences. Such a principle,, 

 I think, we have in the metamorphosis of embryonic life. 

 Indeed, if it can be shown, that besides the differences which 

 exist in all fishes between their earliest forms and their full 

 grown state, there are peculiarities in sharks, skates, and 

 lamprey eels, common to all of them, from an early period of 

 development, which remain characteristic throughout life, 

 it must be acknowledged that these families belong to one 

 and the same great group, notwithstanding their extreme- 

 differences in their full-grown condition. Now, such facts^ 

 exist. In the first place, it is impossible, without disturb- 

 ing their true affinities, to consider an exti'aordinary de- 

 velopment of pectoral and ventral fins as a standard to 

 appreciate fundamental relations between fishes, as in all 

 fishes^ without exception, they are both wanting in earlier 

 life, and as there is scarcely a family in which ventrals 

 at least, are not wanting in some genus or other. We 

 might just as well place Petromyzons among the eels, as 

 their common English name purports, on the ground of the 

 deficiency of their abdominal and thoracic organs of loco- 

 motion, as separate them from the other Placoids. Again^ 

 the peculiarities in the development of the dorsal, caudaU 

 and anal fins in sharks and skates, and the difference which 

 exists between them and the Petromyzons, indicate in no 

 way their affinity or their difference ; in Petromyzon we have 

 the embryonic condition of vertical fins, where a continuous 

 fold in the skin of the middle line extends, as in all embryo 

 fishes, from the back round the tail, towards the abdominal 

 region. In the sharks we have distinct vertical fins, as they 

 generally grow out of the continuous embryonic odd fin • 

 whilst in the skates these fins disappear almost entirely, or 

 are considerably reduced. That animals in their embryonic 

 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 shew that these animals do not belong to the same na- 



