No. 433.] CERTAIN GROUPS OF BIRDS. 



6l 



the articular face for the quadrate, and the extremity is 

 obliquely truncated " (Marsh). Hesperornis regalis possessed 49 

 vertebras in its vertebral column, or 23 presacrals, 14 sacrals, 

 and 1 2 caudals. Both in the articular facets of the centra and 

 in other characters they agree with such modern genera as 

 Urinator and other divers. In its caudal skeleton this great 

 cretaceous diver was peculiar, the free anterior caudal vertebrae 

 being short, with lofty neural arches and not conspicuous 

 diapophyses, and exhibited an opisthoccelian articulation. Great 

 horizontal expansion characterized the long transverse processes 

 of the mid and posterior caudals, while the last three or four 

 elements of this part of the vertebral chain coossified together 

 in the adult, forming a flat, horizontally compressed, pygostylous 

 mass, very different from anything to be found in the form of 

 the terminal piece in the tail of existing Aves. None of the 

 presacral vertebrae united, and none of the caudals possessed 

 zygapophyses. It is very likely that Hesperornis used its 

 broad, horizontally flattened tail much as the now-existing 

 beaver among modern mammals employs its paddle-like caudal 

 appendage, — a powerful aid as a propeller and rudder to the 

 aquatic locomotion of this ancient pygopodous fowl. 



The sternum in this genus is flat and broad and thin posteri- 

 orly. Anteriorly, it is rounded and projecting, while but two 

 shallow notches are to be seen in its xiphoidal margin. The 

 ribs, some of them bearing uncinate processes, resembled those 

 of the loons, but the shoulder girdle, with its non-united clavicles, 

 was weak and small in character, and the pectoral limb was 

 reduced to a rudimentary humerus. 



The pelvis of Hesperornis was like the pelvis of our modern 

 loons and grebes, and Marsh observed that it resembles that 

 of Podiceps, being very long and narrow, as in that genus, and 

 in other diving birds. He also remarked that the " acetabulum 

 differs from that in all known birds, in being closed internally 

 by bone, except a foramen that perforates the inner wall, as in 

 the crocodiles. The ilium, ischium, and pubis, moreover, have 

 their posterior extremities free and distinct." 



The powerful bony framework of the pelvic limbs of this great 

 extinct diver agrees in many characters with the corresponding 



