252 LAKE SUPERIOR. 



acknowledged that Cyclostomata and Plagiostomata are only different 

 degrees of one and the same great type. Now, such characters we 

 have ; m the first place, m the structure of the mouth, which differs 

 so widely from that of the other fishes, and agrees so closely in all 

 Placoids, as Muller himself has shown in his Anatomy of Myxinoids. 

 Next, the teeth also agree, in being arranged in several concentric 

 series, and also in their microscopical structure, as well as in their 

 mode of attachment to tlie skin lining the jaw, and not to the bone 

 itself. We have other hints of the relation between Cyclostomes 

 and Plagiostomes .in their spiracles, and also in their numerous respi- 

 ratory apertures, so that, after due consideration, I come to the con- 

 clusion that the IMyxinoids and Petromyzons, far from being the types 

 of pecuhar subclasses, are simply embryonic forms of the great type 

 to which sharks and skates belong, bearmg to these powerful ani- 

 mals, in a physiological point of view, the same relation which exists 

 between Ichthyodes and the tailless batrachians. 



Of Cyclostomata, two species have been mentioned as occurring in 

 the colder parts of North America, both referred by Dr. Richardson 

 to the genus Petromyzon proper, but of which I have seen no trace 

 myself in the great lake region, though I know Petromyzons to occur 

 below Niagara Falls, However, I am able to add a new species of 

 this family to the fauna of those waters, which belongs to the genus 

 Ammocoetes, and was found in the mud in Michipicotin River, at the 

 landing place of the Factory, the first specimens of which were picked 

 up by the students when dragging their canoes along the shore. 



Ammoccetes borealis, Agass. 



This pretty little species differs from all those already known, by 

 easily appreciable characters. It is at first sight plainly distinguished 

 from the Ammocoetes hicolor., Les. and A. brancJiialis, Dum. whose 

 dorsal fin is, as it were, divided into two lobes by a very low emargina- 

 tion ; but it resembles the Am. concohr, Kirt. and unicolor, Dekay, 

 in its dorsal fin, being uniformly continuous. It differs, however, from 

 this latter, whose form is much moi*e elongated, by the extent of its 

 dorsal fin, which equals one half of the whole length of the body, whilst 

 in the Am. unicolor it extends scarcely before the anus. In the 



