224 PEOF. J. B. GREENE ON THE MUTUAL BELATIONS 



l'\ has been well said that the characters of groups are only a 

 declaration of their prevailing tendencies. " Botanists," writes 

 Mr. Bentham (and his words apply .with equal force to the 

 zoologist), " cannot be so mathematically exact as geographers ; 

 and where an isthmus is very narrow, we must class the peninsula 

 with the island. How often does it happen that two large orders, 

 say of 500 to 2000 or 3000 species, totally distinct from each other 

 in all those species by a series of constant characters, are yet con- 

 nected by some small isolated genus of a dozen, half a dozen, nay, 

 a single species, in which these very characters are so inconstantj 

 uncertain, or variously combined as to leave no room for the strait 

 through which we ought to navigate between the two islands!" 

 In like manner, it might be said that JLepidosiren and A7'chego- 

 saurus form two very narrow but short bridges, which lead from 

 the low-lying country of Fishes to the higher ground of the 

 Amphibians. 



To the preceding remarks it has been thought desirable to 

 append analytical arrangements of the three primary groups of 

 cold-blooded Vertebrata, each of these, for convenience of reference, 

 being regarded as a separate class. 



Under Beptilia* may be included the four orders usually ad- 

 mitted by zoologists, together with most of the extinct orders de- 

 fined by Professor Owen. The claim of CrocodiUa to be viewed as 

 a distinct order, though founded on jvist grounds, is not universally 

 recognized. 



To AmpJiibiaf may be referred the extinct orders Ganocephala 

 and Lahyrintliodontia, together with all the recent HatrachiaX- 

 These last, however, constitute more than a single order, even in 

 the opinion of those, such as Stannius and Van der Hoeven, who 



* The terms Pholidota (Merrem), Monojpnoa (Fitzinger), and Haplopnoa 

 (Van der Hoeven) are synomymous with Reptilia m its proi^er signification. 



t Smce the time of Limieus, who placed all the cold-blooded Vertebrates, 

 except the osseous Fishes, in liis class Amphibia, some natm'aUsts have contmued 

 to use this term in a very general sense, so as to include both the true Amphi- 

 bians and the ReptUes. The late Prince Bonaparte (in liis " New Systematic 

 Arrangement of Vertebrate Animals," Linn. Trans. 1841) and Stannius (Hand- 

 buch der Zootomie der Wirbelthiere, zweiter TheU, zweite Auflage, BerUn, 1856) 

 may be cited, among the moderns, as examples. Dipiioa (F. S. Leuckart) and 

 Diplopnoa (Van der Hoeven) are synonyms of Amphibia proper. 



J The term Batrachia is most frequently employed as synonymous with 

 Amphibia ; but Stannius, Van Beneden, and Gervais restrict its application to 

 the Anura, 



