360 



STRUCTURE AND CLASSIFICATION OF BIRDS 



a thin median line. The interorbital septum is very imper- 

 fect. Basipterygoid processes are absent in the adult, but 

 present as rudiments in the young. The lacrymals are 

 firmly united with the prefrontals, as in gulls and Limicolse. 

 The vomer is distinctly double in the young. Alca tor da 

 has an os uncinatum. In Uria troile and in Alca tor da and 

 A. impennis there is on each side a foramen at anterior end 

 of supra-orbital grooves, as in Chionis, &c. There are fifteen 

 cervical vertebra ; the first few dorsal vertebrae have very 

 conspicuous haemapophyses, bifid at their free ends. Seven 



FIG. 173. TEXSORES PATAGII OF L/inda 



cirrliata (AFTER BEDDARD FROM FORBES). 



o, slip to ulnar side of fore arm. 



FIG. 174. THE SAME OF Synthll- 

 borliainpli us antiq it its. 



ribs reach the very long and narrow sternum, which has one 

 notch on each side posteriorly (Uria), or in addition a fenestra 

 on each side and a median notch (Fratercula) or no notches 

 at all (Alca impennis). 



As to the muscular anatomy of the auks, the simplest 

 form of the ten sores pa'tagii tendons l is seen in Alca tor da, 

 where there are two brevis tendons, both of which pass over 



1 For muscles of Alca (and Spheniscus) see A. CARLSSOX, ' Beitriige zur 

 Kenntniss d. Anatomie d. Schvfimmvogel,' Bill. K. Svensk. Vet. Ak. Handl. 

 ix. 1884, No. 3. 



