418 PROF. HUXLEY ON THE CLASSIFICATION OF BIRDS. [Apr. 1 1, 



organs of this kind than their congeners, accompanied by the cor- 

 relative hot blood. 



But since we know that the organs of respiration and circulation of 

 a Bat are very different from those of a Bird, it is quite possible that 

 those of a Pterodactyle may have been different, in detail, from either. 



Having thus arrived at the conclusion that the class Aves, while 

 well enough defined from all existing Reptiles, is nevertheless far 

 more closely connected with the class Reptilia than with any other, 

 I proceed to inquire how Birds may be svdjdivided into orders, sub- 

 orders, and families, by characters equalling, or at any rate approach- 

 ing, in definiteness those which mark out the corresponding groups 

 among Mammals and Reptiles. 



I propose to divide the class Aves into three orders : the Sau- 

 ^ 4 -^ RURiE, the Ratit^, and the Carinat^e. 



I. The Saurur^ (Haeckel) are represented by the solitary fossil 

 ArchcBopteryx, which seems to have been distinguished from all other 

 birds by the following characters : — 



1 . The metacarpal bones are well developed, and are not anchy- 

 losed together. 



2. The caudal vertebrae are both numerous and large, so that the 

 caudal region of the spine is longer than the body, whereas in all 

 other birds it is shorter than the body. 



The furculum is complete and strong, and the foot extremely pas- 

 serine in appearance. The forms of the skull and of the sternum 

 are unknown*. 



II. The RatitjE (Merrem), or the Struthious Birds, differ from 

 all others in the combination of the following peculiarities : — 



1 . The sternum is devoid of a crest, and ossifies only from lateral 

 and paired centres. 



2. The long axes of the adjacent parts of the scapula and coracoid 

 are parallel or identicalf. The scapula has no acromial process, nor 

 has the coracoid any clavicular process ; at most there are incon- 

 spicuous tubercles representing these processes. 



3. The posterior ends of the palatines and the anterior ends of 

 the pterygoids are very imperfectly, or not at all, articulated with 

 the basisphenoidal rostrum, being usually separated from it, and 

 supported, by the broad, cleft, hinder end of the vomer. 



4. Strong " basipterygoid " processes, arising from the body of 

 the basisphenoid and not from the rostrum, articulate with facets 

 which are situated nearer the posterior than the anterior ends of the 

 inner edges of the pterygoid bones. 



5. The upper, or proximal, articular head of the quadrate bone is 

 not divided into two distinct facets. 



* The " retention of two unguiculate digits on the radial side of the metacarpo- 

 phalangeal bones modified for the attachment of the primary quill-feathers " 

 (Fhilosopliical Transactions, 1863, p. 40) is no distinctive character of Archxso- 

 pteryx, both Struthio and Bhea presenting " two unguiculate digits " in the manus. 



t My friend Professor Newton informs me he had already drawn attention to 

 this important point in his Lectures delivered at Cambiidge last autumn. 



