xliv 



On the assumption that the Gymnonoti, the Scyphophori, and the 

 Nematoqxatiii on the one hand, and the Apodes on the other, are deriva- 

 tives from the Physostome Teleocephals or their immediate progenitors, 

 they should, perhaps, be projected after the Teleocephals as successively 

 more differentiated offshoots, but for the present, at least, it is deemed 

 advisable to retain them in the customary position ; it is to be understood, 

 however, that they form a diverging line from the supposed common 

 stock, and hence the sequence adopted in the list of families. 



In addition to the orders here mentioned, several others appear to be 

 represented by extinct fishes, but we are not sufficiently acquainted with 

 the details of their structure to introduce them with certainty in the sys- 

 tem. It may be suggested, however, that one of the orders is constituted 

 of the Placoganoidei (when restricted to such forms as Pterichthyidee 

 and Goccosteidse) ; another is represented by the triassic and cretaceous 

 Ganoids with a persistent notochord, ordinary pisciform proportions, and 

 non-lobate pectoral fins, such as the Caturidae. Further details respecting 

 at least the scapular arch and pectoral limb (probably erroneously restored, 

 for the latter, by authors) are requisite before their exact relations can be 

 understood. 



FAMILIES. 



The families have been much multiplied, and, it may be urged, unduly 

 so, and such may really be the case, but as analysis should precede syn- 

 thesis, and as many of the more comprehensive families have either not 

 received diagnoses common to and at the same time peculiar to all their 

 constituents; or, in case of applicable diagnoses, the characters are of 

 suspicious value, it has been deemed best to isolate the groups as families, 

 and allow them to stand on their own merits. Several of the families 

 admitted (e.g., Gadiform, Labyrinthiciform, Scombriform, Perciform, 

 Siluriform), are, however, of very dubious value, and are only provi- 

 sionally adopted and kept in prominence to attract future examination. 



There will doubtless always exist more or less difference of opinion as 

 to the taxonomic values of groups, and all that can be hoped for is essen- 

 tial concurrence of views as to the mutual relations of the various 

 groups and their respective degrees of subordination. Ichthyology has 

 not yet, however, reached that stage wherein even an approximate concur- 

 rence in any of these points is possible ; and it is not to be wondered at 

 that the greatest difference of opinion should prevail with respect to 

 families. Much of this dissent is due to the fact that certain groups 

 stand isolated from others, and the relations inter se of the constituents 

 of such groups are so obvious and evidently suggestive, and contrast so 

 strongly with any other group that, although many and very marked dif- 



