SACRUM AND PELVIS. 21 



SACRUM AND PELVIS. 



The preservation of the sacrum and pelvis of the type of P. inc/ens Marsh, No. 1175, 

 is remarkable, displaying to advantage the principal characters of these important skeletal 

 parts. Plate X exhibits the slightly distorted form of this specimen, while restored 

 diagrams have also been prepared to show the side, top, and bottom views, respectively 

 (Plate XI, figures 1, 2, and 3). 



Ten vertebras firmly anchylosed together constitute the sacral series, using this term 

 in its broader sense the " synsacrum " of Professor T. J. Parker. The entire length 

 of the inflexible column so formed is 184 mm . The extremities of the neural spines of 

 all these vertebrae are united by a continuous median longitudinal band about 9 mm 

 wide and about 6 mm in vertical depth. In general form, the three anterior members 

 of this series are similar to the free dorsals which precede them. Their transverse 

 processes arise from the sides of the neural arch above the level of the centra and are 

 fused distally with the anterior blades of the ilia. The first of these vertebras bears 

 anterior zygapophyses for articulation with the last free dorsal, and the transverse 

 processes of these three anterior vertebrae have on their lower surfaces small facets for 

 the support of ribs, as is frequently the condition in the avian sacrum. These facets are 

 most distally situated on the transverse processes of the first vertebra, least so on those 

 of the third. What appears to be the proximal portion of one of these ribs still lies 

 upon the third vertebra with little displacement from its original position. The distal 

 ends of the transverse processes of vertebra 4 extend downward and backward as stout 

 buttresses, which support the ilia more broadly than do the transverse processes of the 

 preceding and of the following vertebrae. The transverse processes of vertebrae 5, 6, 

 and 7 are separated by much smaller foramina, and they unite distally to form a con- 

 tinuous support for the ilia, as in some herbivorous Dinosaurs. The three remaining 

 vertebrae, numbers 8, 9, and 10 of this series, bear shorter transverse processes arising 

 more nearly at the level of their centra; they are separate at their distal ends, which 

 connect with the ilia. Their neural spines also aid in supporting the pelvis. 



The ilia extend forward as broad thin plates resting against the transverse processes 

 of the anterior sacral vertebrae. Posteriorly, they unite with the transverse processes of 

 vertebrae 8, 9, and 10. They also converge medially on the dorsum and fuse with the 

 median longitudinal band over the neural spines of vertebrae 8, 9, and 10. So complete 

 is the anchylosis of these parts that they can not be differentiated with any degree of 

 certainty. 



The united pubes and ischia are directed downward and backward, and meet below 

 in a median symphysis. The obturator foramina lie just beneath the imperforate ace- 

 tabula. They are circular in form, of about half the diameter of the acetabula, and may 

 be considered as marking the theoretical line of fusion between the true pubic and 

 ischial elements. On the anterior border of these ischio-pubic expansions are small 

 facets, which undoubtedly served for the attachment of the prepubes. The prepubes of 

 this example have not been preserved, and in no specimen in the Marsh Collection 

 have they been found in place. Plate XVI, figure 3, shows the prepubis belonging to 

 Pteranodon sp., No. 2472, which is about the size of No. 1175. 



The remarkable similarity of Pteranodon's sacrum to those of many recent birds is 

 distinctly brought out by comparing it with the sacrum of the Wild Turkey, Meleagris 



