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 tarda 

 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 CMonis, &c. There are fifteen 

 cervical vertebrce ; the first few dorsal vertebrae have very 

 conspicuous hsemapophyses, bifid at their free ends. Seven 



Pig. 173. — Tensores Patagh of Lunda 



cirrhata (afteK Beddaed eeom Foebes). 



fl, slip to ulnar side of fore arm. 



Fig. 174. — The Same of SynthU- 

 horhamphus antiquits. 



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 nmscular anatomy of the auks, the simplest 

 form of the tensores patagii tendons ^ is seen in Alca tarda, 

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



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

 Kenntniss d. Anatomie d. Schwimmvogel,' Bih. K. Svensk. Vet. Ak. Eandt. 

 ix. 1884, No. 3. 



