SUPER-ORDER IV EUOENITHES 275 



the Lower Miocene of France, while from the same formation and locality 

 comes Leptoptilus, now confined to warmer regions of Asia and Africa. From 

 the Middle Miocene comes a true heron, Ardea, and above that the species 

 represented are of existing genera. The flamingoes are discussed under the 

 Clienomorphae. 



Order 6. STEGANOPODBS. 



The Steganopodes include the pelicans, cormorants, gannets, frigate 

 birds, and tropic birds, and form a fairly homogeneous group of birds with 

 a desmognathous skull, a sternum with a feeble keel, on the forwardly pro- 

 duced anterior portion of which the lower ends of the clavicles rest and are 

 frequently joined by anchylosis. A distinctive external feature is the union of 

 all foiir toes by a web. 



The desmognathism of this group, as indicated by the species that have 

 been most carefully studied, is of a different nature from that of the Accijntres, 

 being a secondaxy character acquired after the bird has been hatched, and due 

 to the extension of ossification into the palatal region, which finally unites the 

 various portions. In the cormorants, Phalacrocoracidae, and probably in the 

 gannets, Sulidae, this is concomitant with the closing of the external nostrils, 

 for the cormorants when hatched are schizognathous and holorhinal, and this 

 condition lasts up to about the time they take to the water. The changes 

 that occur are directly connected with the bird's habits, for the cormorants 

 pursue their prey beneath the surface, while the gannets plunge down upon it 

 from above. This absence of nostrils is associated with absence of the supra- 

 orbital glands, and this in turn with the lack of the depressions to contain 

 them, which are such obvious characters in the skulls of aquatic birds Avith 

 open nostrils, such as ducks, gulls, petrels, and penguins. 



The Steganopodes seem to have differentiated early, for a cormorant, Gracu- 

 lavus, occurs in the Cretaceous of the United States. Cyphornis magmcs is 

 known from the Eocene of Vancouver, and Prophaethon shrubsolei and 

 Odontopterijx toliapicus from the London Clay, Lower Eocene. It is possible 

 that this last may not belong to the order, although believed to be related 

 to the gannets ; it is distinguished by having the mandible armed with 

 tooth-like projections. The genus Phalacrocorax ranges from the Eocene of 

 Montmatre to the present, and other members of the oilier are found in the 

 Miocene of Europe and Pliocene of the United States. 



Order 7. OPISTHOCOMI. 



The order Opisthocomi contains but a single species, the extraordinary 

 hoactzin (Opisthoconms cristatus) of South America. The skull is schizo- 

 gnathous, without basipterygoid facettes. The sternum is widest posteriorly, 

 and the keel is cut away in front, a peculiarity connected with the presence 

 of a large crop ; the f urculum is anchylosed to the coracoids above and to the 

 sternum below. In the nestling the thumb is large, clawed, and used in 

 climbing, but in the adult it is clawless, and even smaller than in the majority 

 of birds, being a remarkable instance of rapid retrogression. The hoactzin 

 seems to have affinities with fowls, on the one hand, and the plantain-eaters, 

 Muso/jJiagi, on the other, and is considered as a survival of a primitive type, 



