XX 



are the Marsipobranchiates {Lampreys, etc.), and the tendency has been 

 rather to overlook the fundamental differences between the two, and to ap- 

 proximate them too closely, than the reverse. 



PISCES. 



But here unanimity ends, and much difference of opinion has prevailed 

 with respect to the succession in the system of the several sub-classes (by 

 whatever name called) of true Fishes, (1) some (e. g. Cuvier, J. Muller, 

 Owen, Liitken, Cope) arranging next to the lowest, the Elasmobranchiates 

 and, as successive forms, the Ganoids and Teleosteans, (2) while others (e. g. 

 Agassiz, Dana, Dumeril, Giinther) adopt the sequence Leptocardians, Mar- 

 sipobranchiates, Teleosteans, Ganoids, and Elasmobranchiates. The source 

 of this difference of opinion is evident, and results partly from metaphysical 

 or psychological considerations, and partly from those based (in the case 

 of the Ganoids) on real similarities and affinities. 



ELASMOBRANCHIATES. 



The evidence in favor of the title of the Elasmobranchiates to the " high- 

 est" rank is based upon, (1) the superior development of the brain ; (2) 

 the development of the egg, and the ovulation ; (3) the possession of a 

 placenta ; and (4) the complexity of the organs of generation. 



(1) It has not been definitely stated wherein the superior development 

 of the brain consists, and as it is not evident to the author, the vague 

 claim can only be met by this simple statement : it may be added, however, 

 that the brains comparable in essentials and most similar as a whole to 

 those of the Marsipobranchiates, are those of the Sharks. In answer to 

 the statement that the Sharks exhibit superior intelligence, and thus con- 

 firm the indications of cerebral structure, it may be replied that the impres- 

 sion is a subjective one, and the author has not been thus influenced by his 

 own observations of their habits. Psychological manifestations, at any rate, 

 furnish too vague criteria to be available in exact taxonomy 



(2) If the development of the eggs, their small number, and their invest- 

 ment in cases, are arguments in favor of the high rank of the Elasmobran- 

 chiates, they are also for the Marsipobranchiates, and thus prove too much 

 — or too little — for the advocates of the view discussed. The variation in 

 number of progeny among true Fishes (e. g., Cyprinodonts, Embiotocids) 

 also demonstrates the unreliability of those modifications per se. 



(3) The so-called placenta of some Elasmobranchiates may be analogous 

 to that of Mammals, but that it is not homologous (i. e., homogenetic) is 

 demonstrable from the fact that all the forms intervening between them 

 and the specialized placental mammals are devoid of a placenta, and by the 

 variation (presence or want) among the Elasmobranchiates themselves. 



(4) The organs of generation in the Elasmobranchiates are certainly 



