XXVU1 



the Chondrosteans appear to approach the Elasmobranchiates most — on the 

 other hand, in the development of the paraglenal, and the structure of the 

 base of the pectoral fin, they differ less from the ichthyoid Hyoganoids 

 than do the Crossopterygians and the Dipnoans. Nevertheless, they seem 

 on the whole to be the more direct representatives of the lineal succession 

 from the Elasmobranchiates, although doubtless very much modified and 

 different ordinally from the unknown immediate representatives. This has 

 been the view or at least the practice of all ichthyologists except Prof. 

 Cope. 



The eminent naturalist referred to has contended that the Chondrosteans 

 were more nearly related to the typical fishes, and has (1) combined them as 

 well as the Hyoganoids with the Teleosteans in a peculiar subclass (Acti- 

 nopteri), while (2) the Crossopterygians were differentiated as another, and 

 (3) the Dipnoans retained with similar rank. 



The chief considerations, apparently, which induced Prof. Cope to isolate 

 the Crossopterygians and combine the Chondrosteans with the forms re- 

 ferred to, were the result of his study of the pectoral members and their 

 insertion, and the inference therefrom that there was an essential similarity 

 therein between the Chondrosteans and Teleosts, and a fundamental dis- 

 similarity between them and the Crossopterygians. 



Apart from the development of a single or double ceratohyal, which was 

 evidently regarded as of subordinate importance,' the only expressed diffe- 

 rences between Cope's subclasses Crossopterygia and Actinopteri are found 

 in the constitution of the pectoral fins, viz : — 



Crossopterygia: "Limbs having the derivative radii of the primary 

 series on the extremity of the basal pieces, which are in the pectoral fin 

 metapterygium, mesopterygium, and propterygium." 



Actinopteri : " Primary radii of fore limb parallel with basilar elements, 

 both entering the articulation with scapular arch. Basilar elements re- 

 duced to metapterygium and very rarely mesopterygium. Primary radii 

 of posterior limb generally reduced to one rudiment." 



The question arises (1) whether the fundamental differences exist which 

 appear to be expressed by the definitions cited ; (2) the correlated one, whether 

 too much importance may not have been attached to superficial relations 

 of parts, and too little to fundamental homological relations, and (3) even 

 if the homological relations are as dissimilar as the definitions would indi- 

 cate, are they coincident with others, and thus really indicative of such 

 high value. 



EXCURSUS ON THE PECTORAL LIMB. 



The diagnoses jn question seem to be partly (i. e., the articulation or not 



' The Dipnoi have a double ceratohyal. 



