INTRODUCTION. 



In the following pages the attempt is made to bring to- 

 gether the information which we possess as to the charac- 

 ters of the divisions of the Vertebrata above families which 

 are available for the determination of their relations by the 

 paleontologist. These characters, which are of necessity 

 those of the hard parts, 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 are close enough to render our inferences from the 

 former generally safe. Fortunately, also, the living rem- 

 nants 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 Lepi- 

 dosiren, 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 of 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 synop- 

 sis to bring these definite characters 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 rela- 

 tions of the earliest and the lowest Vertebrata by a want 

 of knowledge of their structure, and by the absence from 

 our collection of numerous intermediate forms which must 

 have existed. Until our knowledge is more complete, the 



