264 



NEW YORK STATE MUSEUM 



Fig. 6 Right lateral view of the pelvis, cau- 

 dal vertebrae, and sacral ribs of Mergus 

 serrator. Natural size, and drawn by 

 the author from specimen no. 16626, United 

 States National Museum Collection ; same 

 skeleton as figures 1-5 



in Mergus, simply somewhat 

 modified in concordance with 

 its life as a diver. 



The ribs of the first three 

 vertebrae that anchylose in 

 the sacrum have already been 

 described when speaking of 

 these bones in general. Next 

 to them we find that the three 

 succeeding vertebrae throw 

 out their apophyses to the 

 pelvis and firmly anchylose 

 therewith. After them we 

 fall into the deep and oblong 

 pelvic basin possessed by this 

 bird, and the next three verte- 

 brae send their processes di- 

 rectly upward. They are fol- 

 lowed by a series of eight 

 more that gradually approach 

 the free caudals in form. 

 The anterior one of these has 

 the strongest lateral processes, 

 but they are found to abut 

 again* t the ilia on either side 

 at a point anterior to the mid- 

 dle of the ischiac foramen, 

 and not right behind the coty- 

 loid cavities as in many other 

 birds. The inner margins of 

 the ilia anchylose with the 

 outer ends of these sacro- 

 vertebral apophyses, from the 

 acetabula, backward, except- 

 ing the last one. 



Opposite the cotyloid cav- 

 ities we find the enlargement 

 to accommodate that part of 

 the spinal cord where the 

 sacral plexus is thrown off; 

 the openings for the exit of 

 the latter are double, being 

 placed one above the other. 



