74 



ODOXTORXIT Ill's. 



The Sacrum. (Plates X and XI.) 



The sacrum in Hesperornis regalis is very long and narrow, and the 

 vertebra? which compose it are usually well coossified. The number of 

 vertebras in the true sacrum cannot be accurately determined, but in the 

 ankylosed series, illustrated in plates X and XI, there are fourteen. This 

 series may be conveniently described as the sacrum, although some of the 

 anterior vertebrae are evidently sacro-dorsals, and some of the posterior 

 should be regarded as m-osacrals or caudals. 



The first vertebra of the series is the twenty-fourth in number, count- 

 ing from the skull. It is the largest of the sacral series, and is entirely 

 enclosed between the ilia. Its transverse processes bear no articulated rib, 

 but are expanded at the distal end, and abut directly against the ilium. 

 The line of union of this vertebra with the succeeding one is strongly 

 marked, and in one specimen of this species (number 1477), the vertebra 

 is free, and hence it is represented in the figures below, and its measure- 

 ments are given separately. The junction between the twenty -fifth and 

 twenty-sixth vertebras can be traced on the lower siu~face, but behind this 

 the lines of union are obliterated, except two or three near the posterior 

 end of the series. 



The figures below represent the twenty-fourth vertebra taken from 

 the series in which it is distinct. 



Figure 21. — Twenty-fourth vertebra of Mesperornis regalis, Marsh; (No. 1477) front view; natural size. 

 Figure 22. — The same vertebra; posterior view. The neural spine in this specimen is not preserved. 

 d. diapophysis ; nc. neural canal ; z. pre-zygapophysis. 



