152 Mr. P. R. Lowe on the 



distinctly diflferentiated in various minor details. The 

 relative measurements are also different, as shown below : — 



Length of femur in Chionis alba 60 mm. ; in H. ostralegus 50 mm. 



„ tibio-tarsiis „ „ 81 mm. ; „ „ 86 mm. 



„ tarso-metatarsus „ „ 43 mm. ; „ „ 55 mm, 



„ middle toe „ „ 38 mm. ; „ „ 41 mm. 



Vertebral Column. — In the Sheath-bills there are only two 

 cervico-dorsals ; in Hamatopus there are three ; while the 

 morphology of the hypapophyses of the cervical vertebra in 

 the two forms is strongly contrasted. 



Summaj'y. 



The sum of the characters presented by the skeletal, 

 pterylographical, and other features of the Chionididse point 

 to the fact that this very specialised and well-defined 

 Cbaradriiform group is more pluvialine than larine. It is, 

 however, so specialised away from tiie "Plovers'" that its 

 inclusion in the limicoline suborder (Charadriidse+Scolo- 

 pacidte) seems a matter of doubtful propriety. 



Kidder & Coues (Bull. U.S. Nat. Mus.) thought that 

 Chionis was a connecting-link closing the narrow gap 

 between the Plovers and Gulls of the present day. In 

 their opinion the Sheath-bills repi-esented the survivors of 

 an ancestral type, from which both the Gulls and Plovers 

 have descended. In this opinion I tbink there can be 

 no doubt that they were mistaken, since, among other 

 reasons, the Sheath-bill is not a generalised type but a 

 specialised one. It is probably nearer the truth to suppose 

 that the Sheath-bills were differentiated as an offshoot from 

 the main cbaradriiform stem before that stem had split 

 into the charadriine and scolopacine branches, and that 

 that offshoot was given off prior to the differentiation of the 

 Skuas and Gulls ; or, as an alternative speculation, that 

 the main cbaradriiform stem split into a limicoline and a 

 laro-limicoline branch — such groups as the Sheath-bills, 

 Crab-Plover, Pratincoles, Skuas, Gulls, Terns, and Auks 

 arising from the latter by various stages of specialisation. 



