and on some Characteristics of the Reptilian Birds. 17 



and- salamanders and other included species having gills like fishes, 

 besides some additional fish-like peculiarities. 



The parallelism between the three classes, Mammals, Birds, and 

 Reptiles, is thus complete. 



(4.) Fishes have no class of Vertebrates below them, so that 

 an in/enor hemitypic division is not to be looked for. It might 

 be suspected that the intermediate group in this case would be 

 one between Fishes and the lower sub-kingdoms either of Mollusks 

 or of Articulates ; but none such exists. The lowest fish, an 

 Araphioxus, is as distinctly a Vertebrate as the highest, and no 

 Mollusk or Articulate exhibits any transition towards a vertebrate 

 structure. 



There are, however, hemitypic Fishes ; but their place is towards 

 the top of the class instead of at its bottom. Ganoids constitute 

 one group of this kind, between Fishes and Reptiles, as long since 

 pointed out by Agassiz. Again, Selachians (or Sharks and Rays) 

 constitute another, between Fishes and the higher classes of Ver- 

 tebrates. This last idea also has, we believe, been suggested by 

 Agassiz (although we cannot refer to the place wdiere published), 

 this author regarding the species as intermediate in character be- 

 tween Fishes and the allantoidian Vertebrates. Moreover, Miiller 

 long ago observed the relation of the Sharks to the Mammals in 

 having a vitelline placenta, by which the embryo draws nutriment 

 from the parent, as does the mammalian foetus by means of its 

 allantoidian placenta. 



Ganoids and Selachians are thus two hemitypic groups in the 

 class of Fishes. 



The scheme of grand divisions is then as follows : — * 



I. 



A. Typical Mammals. 



B. Hemitypic Mammals, 



or OoTOCOIDS. 



II. III. 



A. Typical Birds. A. Typical or true Reptiles. 



B. Hemitypic Birds, B. Hemitypic Reptiles, 



or Erpetoids. or Amphibians. 



IV. 



A. Hemitypic Fishes, B. Hemitypic Fishes, 

 or Selachians. or Ganoids. 



C. Typical Fishes, 

 or Teliosts. 



One of the groups of hemitypic Fishes looks directly towards 



* It is here seen that the term Ootocoid, applied to Marsupials and Mono- 

 tremes has great significance ; and so likewise, Erpetoids and Amphibians. 

 Ootocoid is simply the Greek form of the term semi-oviparous. 



