CLASSIFICATION OF VERTEBRATA 



•know" that the palaeozoic Fishes did possess an entirely 

 venous heart, nor has it yet been shown that the embryos 

 of Dinosaurs were surrounded by an amnion; but we feel 

 nevertheless certain, because .of the laws of correlation win eh 

 comparative anatomy allows us to deduct from the study of 

 recent creatures. On the other hand, it is quite possible, 

 even most likely, that the triassic Pseudosuchia, p. 19, had 

 no copulatory organ, and therefore this feature cannot be 

 admitted into the diagnosis of Crocodilia, at least not if they 

 are to comprise the Pseudo-, Para-, and Eusuchia. 



The various characters employed are, of course, not all 

 equivalent. The same character, which in some groups is 

 scarcely of more than generic value, runs perhaps through all 

 the members of another class. 



The groups into which we are used to combine the animals 

 of the various classes are not, and cannot be, all equivalent. 

 The least objectionable, or rather the most generally accepted 

 " orders," are those of the Mammalia, and it is well understood 

 that the ornithologists' "orders" are of far less morphological 

 value, while the time-honoured "orders" of Reptilia are 

 of infinitely greater importance. 



Each class has, so to say, its own standard units, jut 

 <>ne nation reckons with £ s. d., another with dollars and 

 cents, and a third with Mark and Pfennige, which again are 

 not the same as francs and centimes. However, to mitigate 

 the discrepancies as much as possible, and chieily owing to 

 the bewildering mass of fossil reptiles which have come to 

 light, I have arranged the reptiles in numerous sub-classes, 

 and these again in orders, while for the host of Fishes, 

 divisions," and for the Birds "divisions," and "legions" 

 have been resorted to as intermediate groups between sub- 



