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THE AMERICAN NATURALIST. [Vol. XXXVII. 



anatomist, D'Arcy W. Thompson, and myself have carefully 

 examined into the osteology of these extinct divers, and have 

 shown that they undoubtedly are among the ancestors of the 

 existing Pygopodes (vide posted). 



The skull of Hesperornis in its general formation resembles 

 closely the skull of such a diver as Urinator lumme, but exhibits 

 characters common to many birds belonging to widely sepa- 

 rated groups of the age in which it lived. For example, the 

 capacity of the cranial casket was small ; the superior articular 

 head of the quadrate bone had but one articular facet ; they had 

 teeth in grooves, all the length of either ramus in the lower 

 jaw, but confined to the dentary borders of the maxillaries in 

 the case of the upper. The ramal symphysis was cartilagi- 

 nous throughout the life of the individual, and consequently 

 each ramus was a separate bone. 



According to Marsh, the proximal extremities of the palatines 

 and the distal ends of the pterygoids did not articulate with 

 the basisphenoidal rostrum. The stout basipterygoid processes 

 arose from the body of the basisphenoid, and each one articulated 

 with a facet upon the corresponding pterygoid situated near 

 its proximal end. It had separate vomers. The supraorbital 

 glandular depressions, the holorhinal nostrils, the firmly united 

 intercranial sutures, the form of the quadratojugal, and many 

 other characters, exhibiting but little change in their now living 

 descendants, are each and all characteristic of the Pygopodes. 

 As the teeth do not belong to the skeleton, they require no 

 special description in an article devoted to osteology ; be it 

 sufficient to say that they were purely of a reptilian type. 



The mandible was long and slender, and "the rami were 

 united at the symphysis in front only by ligament, a feature 

 unknown in modern adult birds. There is an imperfect articu- 

 lation between the splenial and angular elements, which prob- 

 ably admitted of some motion, and all the other sutures are 

 open, or distinguishable. There was apparently a mandibular 

 foramen. There is a well-marked shallow groove on the outer 

 superior margin of each dentary bone for the reception of the 

 maxillary teeth when the jaws were closed. The angle of 

 the mandible extends backward but a short distance beyond 



