312 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [ToLYl. 



The lateral bodies of this bone are cellular masses with several aper- 

 tures opening forwards and outwards, the mastoids closing them in, in 

 the completed cranium. On its outer surface near the inferior angles 

 we observe two, one on either side, grooved foramina, leading upwards 

 and inwards, to open into the lateral sinuses on the inner surface of the 

 segment, nearer together. As age advances these canals contract, but 

 still exist throughout life. 



The basioccipital segment, bo, also is largely cancellous in structure, 

 wedged shaped, having at its apex a long, rounded tubercle curving 

 outwards and backwards, overhanging a slight depression beneath it. 

 This tubercle in the comj)lete vertebra forms the middle two-thirds of 

 the occipital condyle, which, in the adult, is found below the foramen 

 magnum, sessile, uniform in outline, with the rounded border below, and 

 all indications of its original division into three parts obliterated. 



The neurapophyses of this vertebra, termed the " exoccipitals " (Plate 

 V, Fig. 51, eo), are each nearly as large as the neural spine ; on their inner 

 borders they present for examination the deeply-rounded margins to 

 corapkite the foramen magnum, and immediately beneath, the minute 

 tubercle jutting out that lends its assistance on either side to form the 

 condyle of the occiput. 



The outer angles, quadrate in outline, deflected slightly downwards, 

 are the transverse processes of the vertebra, the " i)aroccipitals." The 

 precondyloid foramina are also to be observed here, with one still more 

 external, belonging to the group from which the eighth nerve makes its 

 way from the cranium. The internal aspect of an exoccipital is a mass 

 of open, irregular cells, that are closed in when this segment is approxi- 

 mated with the mastoid, superoccipital, and the " petrosal " (Fig. 51, 1), 

 that odd-shaped and spongy bonelet which constitutes the capsule of 

 the organ of hearing — and which has a foramen on its inner and smooth 

 surface for the passage of the auditory nerve — forming, also, by a bend- 

 ing forwards of a part of this surface, and aided by the basi-sphenoid, 

 the floor of the mesencephalic fossa on either side, while externally it 

 shares in forming the entrance from without to the otocrane. 



With the exception of the petrosal, the elements thus far described, 

 when duly articulated, form the neural arch of the occipital vertebra, as 

 already intimated above. The basioccipital, the centrum of this verte- 

 bra, by its larger extremity, and the exoccipitals with the connate 

 diapophyses articulate with the basi-sphenoid in the basi-cranii below; 

 the latter, with the superoccipital, meet the parietals and mastoids above 

 and laterally. In old birds every trace, both sutural and otherwise, 

 becomes completely obliterated as the osseous amalgamation pro- 

 gresses, though throughout the group a well-defined "superior line" 

 limiting muscular attachment, indicates very nearly the terminating 

 borders above, and sometimes, as in Centrocercns, a fainter indication 

 exists in the vicinity of the union among the interested bones below. 

 On either side of the condyle, to its outer aspect we observe in a slight 



