548 P. H. EDGEWORTH. 



No one of tlie Sauropsida has preserved all these features, 

 but ill each group they exist (though sometimes masked) in 

 embryonic stages, and the various chauges which take place 

 and bring about the characteristics of each can be followed 

 in the subsequent development. 



Birds diifer from all living Reptiles in possessing the 

 following features. The ptery go-quadrate is movable ; there 

 is a joint between the basihyal and the first basibranchial, 

 and a second basibranchial is generally present ; the retractor 

 oculi divides into pyramidalis and quadratus nictitantis ; the 

 upper portion of the mandibular myotome, after giving off 

 the depressor palpebrse iuferioris, forms an elevator or eleva- 

 tors of the pterygoid process of the quadrate ; a distinct 

 quadrato-maxillaris is formed ; the genio-hyoid, after having 

 had a temporary insertion into the first basibranchial, 

 atrophies ; the sterno-hyoid becomes, secondarily, inserted 

 into the dorsal surface of the first basibranchial, and in most 

 Birds divides into anterior and posterior portions ; the hyo- 

 glossus divides into several muscles ; a genio-glossus is seldom 

 developed ; a portion of the posterior mylohyoid generally 

 becomes a separate muscle — the anterior constrictor colli — 

 forming a muscular sling for the second basibranchial; in 

 most Carinatse syringeal muscles are formed. 



Birds resemble the Ehyncocephalia in possessing an upper 

 portion of the mandibular myotome inserted into the ptery- 

 goid process, but the adult condition in the latter group is 

 clearly a secondary modification correlated with a fixation of 

 the pterygo-quadrate. 



The pterygoid process of the quadrate of Birds resembles 

 that of Clielouia in possessing no processus ascendens such 

 as exists in Crocodilia, Ehyncocephalia, Lacertilia vera, and 

 Khiptoglossa, and such a structure (to which no muscles 

 are attached in early stages of development) does not seem 

 to be a part of a freely movable pterygo-quadrate, but rather 

 a formation occurring concurrently with or subsequently to its 

 fixation. 



The condition of the pterygoid muscles in Birds might easily 



