3-56 STRUCTURE AND CLASSIFICATION OF BIRDS 



Subfamily II. Stercorariinae. 



Muscular formula, AXY +. Caca long. Biceps 

 slip present. Expansor secundariorum absent. 



Subfamily III. Sterninge. 



Muscular formula, ABXY+. Caca rudimentary. 

 Biceps slip present. Expansor secundariorum 

 absent. 



Subfamily IV. Rhynchopinse. 



Muscular formula, ABXY-. Cfsca rudimentary. 

 Biceps slip and expansor secundariorum absent. 



Gygis and Anous require further investigation before they 

 can be placed in this system ; they are usually regarded as 

 terns by HOWAED SAUNDEES, for example. Anous has the 

 complete muscle formula. Gygis has the formula of the 

 gulls; but then the tern, Sternula (sp.), wants the accessory 

 femoro-caudal, and in this approaches the gulls. Anous has 

 the expansor secundariorum. 



Of undoubted or reputed extinct limicolous birds a number 



have been described. If PaUeotrinya (with three species), from 



North America, is rightly referred by MARSH to this group, it goes 



back to Cretaceous times. Milnea, from French Miocene, is known 



by the humerus. Elorius, known by an imperfect coracoid, a tarso- 



metatarsus, and parts of the skull, seems to belong here. JEijial- 



ornis is considered by LYDEKKER to have been a gull-like bird, 



largely on account of the perforated first phalanx of the second 



digit. 1 Haley ornis has been described from the extremity of a 



humerus and the back of the skull. 



1 A character which we have seen (supra, p. 351, footnote) to be of no 

 account in fixing affinities. 



