THE 
AMERICAN NATURALIST 
SYNOPSIS OF THE FAMILIES OF VERTEBRATA. 
TN the following pages the attempt is made to bring together 
the information which we possess as to the characters of the 
divisions of the Vertebrata above famiUes which are available for 
the determination of their relations by the paleontologist. These 
characters must be of the first importance to the discovery of the 
phylogenies, since the soft parts are unavailable. It is, however, ' 
true that the relations of these to the hard parts are close enough 
to render our inferences from the latter generally safe. Fortunately, 
also, the living remnants of extinct groups are sufficiently 
numerous to enable us to check our studies of the osteology. 
Thus we have the Branchiostoma, the lampreys, the Ceratodus and 
Lepidosiren, the Sphenodon, and the Monotremata, to which to 
refer when we desire to learn approximately the characters of the 
soft anatomy of ancient forms. 
All the characters of the various divisions are not given. In 
fact, when all extinct forms come to 'be known, no division is 
likely to be defined by more than one character. At present 
several characters may be often ascribed to various divisions, but 
one of these will ultimately prove to be the essential one. It is 
the object of the present synopsis to bring these definitive char- 
acters into prominence ; hence they are always stated first. The 
method of keys is adopted as the most perspicuous method 
of exhibiting them. 
We are embarrassed in the endeavor to present the relations of 
the earliest and lowest Vertebrata by a want of knowledge of their 
