SKULL OF ICHTHYORNIS. 123 



Figure 30 represents the skull and cast of the brain-cavity of a Tern, 

 and the figure of Ichthyornis, beside it, is made of the same absolute length 

 for comparison. The Tern was chosen for this purpose because in size and 

 structure of its skeleton it bore considerable resemblance to Ichthyornis. 

 The mode of life, also, of the two birds was probably similar. 



In comparing the brain-cavity of Ichthyornis with that of Sterna, as 

 shown in figures 29 and 30, the strong contrast in size is at once apparent, 

 while the most marked difference is seen in the cerebral hemispheres. If 

 the latter alone be compared, the hemispheres of Ichthyornis are less than 

 one-fourth the size of those of Sterna. If, however, the bulk of the entire 

 brain of each be compared, that of Ichthyornis would be less than one-third 

 the size of that of the Tern. This would hold true after allowing a con- 

 siderable reduction for any uncertainty in regard to the exact proportions 

 of the posterior part of the brain-case of Ichthyornis. 



The result of this comparison between Ichthyornis and the Tern agrees 

 very closely with that obtained in contrasting the size of the brain-cavities 

 of Hesperornis and Colymbus, as given on page 10 ; and hence is of special 

 interest, as in no other instances have the brain-cavities of Mesozoic birds 

 been investigated. 



The Lowek Jaws. (Plates XXI and XXVI.) 

 In the type specimen of Ichthyornis dispar (No. 1450), both lower 

 jaws are preserved. The two rami are entirely separate, having been 

 united in front only by cartilage. They are large and massive, nearly 

 straight, and much compressed transversely, especially in the posterior 

 portion. The upper margin, of the dentary bone is nearly straight. The 

 anterior extremity is rounded, and without any distinct symphysial sur- 

 face. The right ramus is well shown in Plate XXI, figures 2, 3, and 4. 

 The sutures in this jaw are nearly all obsolete, with the exception of that 

 between the splenial and angular bones. This suture is especially open on 

 the inner surface of the jaw (figure 2, e), and, in life, doubtless admitted 

 of some motion. There is no mandibular foramen, and, just behind the 

 articular face for the quadrate, the extremity is abruptly truncated. 



