xxii 



merit of the skull ; (6) the mode of attachment of the teeth; (7) the slight 

 degree of specialization of the rays of the fins; and (8) the rudimentary 

 condition of the shoulder-girdle. 



In none of these cases is there exact, or even very close similarity, for, 

 as already remarked, the gap between the Fishes (and the Elasmobranchi- 

 ates as the most generalized form) and the Marsipobranchiates is extremely 

 wide. In each case, however, the generalized or rudimentary condition of 

 the organs points to the still more generalized, rudimentary, or undeveloped 

 conditions exhibited by the Marsipobranchiates. The testimony of these 

 parts is also concurrent, is reinforced by other resemblances, less obvious 

 but valuable as accumulative, and is not offset by the evidence of other 

 parts (unless irrelative specialization of isolated parts is considered as con 

 tradictory evidence). And still more, there are no other forms that can be 

 compared with the Marsipobranchiates in even approximately so satisfac 

 tory a manner. Therefore, with no hesitation, the sub-class of Elasmo- 

 branchiates is placed as the succeeding term in the ichthyological series. 



PLA9IOSTOMI. 



On the whole, the Sharks appear to be the most generalized of the Elas- 

 mobranchiates, and there is little doubt but that the Rays are a more 

 specialized offshoot from the same primitive stock. 



HOLOCEPHALI. 



More nearly related to the Sharks than to the Rays, but differentiated 

 from representatives of a primitive line of descent, the HOLOCEPIIALI claim 

 the next consideration. If, in some respects, they appear to be more nearly 

 related to the Ganoids, the Plagiostomes do in others, and it yet remains 

 to be decided which are the most generalized in essential features. Mean 

 while, it seems advisable to preserve the place for the Plagiostomi. 



GANOIDEI. 



By common consent, the Ganoids immediately succeed the Elasmobran- 

 chiates. Before considering the sequence of the forms, a brief inquiry into 

 the constitution of the class may be seasonable. 



HISTORICAL NOTE. 



The name Ganoides (or Goniolepedoti) was originally framed by Prof. 

 Agassiz 1 as an ordinal term for fishes having the scales (when present) 



1 l cr ordre. Ganoides Agass. (Goniolepidoti Agass.). Ecailles anguleuses, rhom- 

 bo idales ou polygones, formees de lames osseuses on cornees, recouvertes d einail. 

 Les families des Lepido ides, des Sauro ides, des Pycnodontes, des Sclerodermes, des 

 Gymuodontes, des Lophobranches, etc. etc. Agass. Recherches sur les Poissons fos- 

 siles, v. 2, p. 1. 



