T. Gill—Number of Classes of Vertebrates. 433 
and the whales, and thus for the first time subordinated habitat 
and adaptation therefor to structure. While at the present day, 
the ancient ideas have almost entirely disappeared from the sys- 
tem of nature so far as regards the terrestrial vertebrates, they 
are still to a great extent prevalent in the appreciation of the 
relations of the aquatic ones. For those vertebrates confounded 
by most naturalists under the name of Fishes, are very dissim- 
ilar among themselves, and so much so even that the differences 
are more marked and radical than those between an e 
superior classes of the branch. If, indeed, considerations’ of 
differences of structure are to guide us in the appreciation of 
the relations and subordination of animals, the current classifi- 
cation must be entirely changed, and the subordination of the 
highest groups, first suggested by Hiickel, should be adopted 
with some modifications, while as respects the combination of 
the “higher” or more specialized classes into superior groups, 
other principles should guide. One of the chief points to be 
reconsidered is the association of the Batrachians with Fishes 
rather than with the true reptiles. Although no distinction may 
be possible between the first two when the class of Fishes has 
the wide range generally allowed, there is no difficulty in their 
discrimination with the limits here to be assigned to them. We 
of auditory organs, the simple tubular structure of the heart, 
and the development of the liver simply as a diverticulum of 
the intestine. This type is called by Hickel the Subphylum 
sequently no lower jaw, there are no pectoral EEE mg 
Scapular girdle, and there is but one nasal sac, which has a me- 
dian external aperture. To this section belong the Lampreys 
and Hags, the representatives of the class of Marsipobranchs. 
Am. Jour. Sc1.—THirpD gage Vou. VI, No. 36.—Dzc., 1873. 
