30 



ture in these birds from personal investii,^ation. Hut in [ \ ivibcr 

 it essentially agrees with what we found in the grebes. 



A complete skeleton of rriuator liDumc (No. 13.646 Smithso- 

 nian Collection) before me, has 43 vertebne in its spinal column. 

 Of these the fourteenth is the first to bear a pair of free ribs ; 

 the succeeding six movable vertebra; connect with the sternum 

 by costal ribs ; the next seventeen unite as a "sacrum'" with the 

 pelvic bones ; then follow six free caudals and a pygostyle 

 containing several more. 



The dorsal ribs are broad, and bear large, freely articulated 

 epipleural appendages. Two pairs of ribs also come off from the 

 sacrum, and meet long, sweeping haemapophyses, that reach the 

 costal borders of the sternum. This specimen has also a " float- 

 ing costal rib," which is very small and delicate. It is seen on 

 both sides. The form of the skeleton of the thoracic parietes 

 agrees to some extent with the shape it assumes in the Alcidae, 

 with its hinder ribs sweeping beneath the pelvis. This latter 

 bone is of extraordinary form and dimensions in all of the 

 Urinatoridae, even excelling the grebes in some of its peculiar- 

 ities. The anterior portion of an ilium is short and depressed 

 in comparison with its extensive backward reach. The neural 

 crest of the sacrum appears above the pelvic bones for its entire 

 length, and posterior to the large elliptical ischiac foramen the 

 ilium looks directly outwards, then outwards and upwards. A 

 small prepubis is present, while the post-pubic element is long 

 and slender, its posterior extremity, curving beneath the pelvis 

 behind, is dilated and paddle-shaped. It nearly meets the fellow 

 of the opposite side, where both are completed by an emargina- 

 tion of cartilage. It differs from the grebes in that it articulates 

 with the postero-inferior angle of the ischium upon either side. 



Five of the last caudal vertebrae, together with the pygostyle, 

 are shown in side view in my above mentioned paper {Jour. Anat. 

 and Physiol). The three first chevron bones there exhibited are 

 freely articulated over the joints of the centra when they are 

 present ; the ultimate ones, however, become anchylosed to the 

 under side of the rear vertebra in each case, the last one really 

 forming the antero-inferior process of the pygostyle. 



Coues in his examination of the skeleton in Urinator imber 



