236 AVES. 



certainly a paired element. The proximal articulation of the single- 

 headed quadrate exhibits a slight division into two facettes of unequal 

 size. There are teeth in both jaws, all arranged in continuous grooves ; 

 those of the mandible extending to the anterior end, but those of the 

 upper jaw confined to the maxilla3. The sutures between the beak and 

 frontal region are persistent, as also are those between the several elements 

 of the mandible and at the mandibular symphysis. The mandibular 

 ramus extends into a considerable process behind its articular facette. 

 There are 23 presacral vertebrae, with centra united by saddle-shaped 

 articulations like those of modern birds. Seventeen of these vertebrae are 

 cervicals, and the last three bear ribs ; all the thoracic ribs exhibit large 

 uncinate processes, and the first four pairs are connected with the sternum, 

 while the two hindermost pairs are free,, long and slender. Fourteen 

 vertebrae are fused together in the much-extended sacrum, and there are 

 twelve caudals, of which the first eight or nine are quite free. The 

 sternum is relatively broad, though elongated and flattened, and destitute 

 of a keel. The coracoid is short and very broad, its long axis continuing 

 that of the free elongated scapula in a gentle curve. The very slender 

 clavicles appear to meet in the middle line without fusion. Of the wing, 

 only the much reduced humerus is known, but this bears articular facettes 

 for the radius and ulna which must have been present. The hind limbs 

 are adapted for swimming, much like those of the modern Colymbidae. 

 The elements of the pelvis are fused together, but the ischium and pubis 

 are very slender and quite free at their distal end ; the pectineal process 

 is conspicuous ; and the acetabulum exhibits a much smaller perforation 

 than is usual in birds. The ilium is anchylosed with the sacrum only in 

 the acetabular region. The femur is remarkably short and stout, and 

 flattened antero-posteriorly. It is not pneumatic, but exhibits a medul- 

 lary cavity. The tibio-tarsus is the largest bone in the skeleton, very 

 long and powerful. It is also hollowed by a medullary cavity. The 

 patella is very large, and separate from the tibio-tarsus. There are the 

 usual four digits in the foot, the metatarsals of the second, third and 

 fourth being firmly fused together in the tarso-metatarsus. The fourth, or 

 outer toe, is much the largest. One specimen shows traces of feathers, 

 which are believed to have been plumulaceous over the whole body, 

 extending even over the tarsus. The genus Hesperornis is known only 

 from the Chalk of Kansas, and the typical species, H. regalis, seems to 

 have attained a height of about a metre when standing. 



Fragmentary bones which seem to represent a bird (Enali- 

 ornis) closely allied to Hesperornis, occur in the Cambridge 

 Greensand, but the pectoral arch and wing still remain un- 

 known. 



