254 PROFESSOR OWEN ON THE GENUS DINORNIS. 



The four ribless sacral vertebree, which in the Dinornis are interposed between those 

 which send their anchylosed ribs to abut upon the os innominatum anterior to the ace- 

 tabulum, and those which strengthen in Uke manner the posterior part of the acetabu- 

 lum, are very short ; their bodies have coalesced into a single mass of bone, smooth and 

 flattened below, rounded at the sides, and only recognizable as distinct bones by the 

 orifices for the nerves at the sides of the anchylosed mass : these orifices are double, 

 as in the sacrum of other birds', the two roots of the nerves escaping separately, the 

 motor root issuing by the lower, the sensitive root by the upper orifice. 



The upper transverse process of the first sacral vertebra is a broad and thick piece 

 of bone, extending from the body and anterior articular process of the vertebra, and 

 having a deep and smooth excavation at its anterior part : in the Ostrich the correspond- 

 ing part is much smaller and is reticulated by the bars of bone dividing the orifices by 

 wliich the air is admitted into the interior of the vertebra. 



I shall not swell the extent of the present paper by pursuing farther the description 

 of the structure of the pelvis of the Dinornis, as exhibited in the present striking frag- 

 ment, but proceed to notice the other specimens of pelvic bones which have been enu- 

 merated as forming parts of the present collection. 



The large portion of the right os innominatum, including the entire acetabulum 

 (PI. XX. fig. 1.), must have belonged to a bird of rather smaller size than the one to 

 which the above-described portion of the sacrum belonged. The part of the ilium 

 before and above the acetabulum rises with a steep slope and a slight general concavity 

 to meet its fellow above the spinous crest of the anterior part of the sacrum : behind 

 the acetabulum the outer surface of the ilium is divided into two facets, the upper 

 one nearly horizontal, the lower one vertical, save where it arches out to the flat arti- 

 cular surface behind the acetabulum. The ridge dividing these two facets commences 

 anteriorly above the middle of the acetabulum, and describes a regular curve in its 

 course backwards, the convexity being downwards : in the Ostrich the correspond- 

 ing ridge forms two curves, meeting at an angle above the prominent articular sur- 

 face behind the acetabulum, and the convexity of both curves is upwards ; from the 

 angle an obsolete ridge extends down to the prominent articulation, and divides the ante- 

 rior from the posterior vertically concave surfaces of the ilium : in the great Dinornis 

 the corresponding surfaces are uninterruptedly continuous above the acetabular promi- 

 nence. The posterior wall of the acetabulum (/) is incomplete, as in other birds ; the 

 smooth articular surface is continued upon an oblong prominence above and behind the 

 cavity. The pubis (rf), a slender bone, as usual in Birds, springs from a protuberance at 

 the lower part of the acetabulum. The ischium (e) is continued more directly from the 

 lower and back part of the cavity : a very slight ridge indicates the posterior boundary 

 of the notch for the tendon of the obturator internus, and the upper border of the notch 



' Cyclop, of Anat., art. Aves, p. 271. The 0.«trich is the only exception to this rule with which I am 

 acquainted. 



