88 BTTLLETIX UNITED .STATES GEOLOGICAL SURVEY. [roJ.VI. 



occipital segments curving ratlier abrux)tly upwards to meet the mas- 

 toids and parietals. All its i)rimary parts are tlioroughly coalesced, 

 and its articulations with the surrounding bones obliterated, save a flue 

 ridge, running transversely, just anterior to the condyle, separated from 

 it by a depression which seems to indicate the remains of the occipito- 

 basi-sphenoidal suture. Posterior to the foramen magnum the bone rises 

 and displays a well-marked "cerebellar prominence", with a depression 

 on either side of it. On the summit of this prominence, in the median 

 line, just before we arrive at the foramen magnum, we And the super- 

 occipital foramen. This foramen varies in size and shape in different 

 indi^^iduals — in size, from one to two millimetres ; in shape, from a circle 

 to a transverse ellipse, though it is usually small and circular. It is 

 said to be formed by a thinning of the bone due to muscular pressure 

 from without and the pressure of the cerebellum from within ; in the 

 fresh specimen it is covered by a thin membrane. Lying in the horizon- 

 tal plane, anterior to the cerebellar prominence, is the foramen magnum. 

 In shape it resembles a square with the four angles rounded off. Its 

 average measurement is five millimetres transversely and four millime- 

 tres antero-posteriorly, the latter diameter being encroached upon by 

 the occipital condyle in the median line. The occipital condyle is sessile, 

 though raised above the level of the basis cranii, hemisplieroidal in form, 

 with a minute notch marking it posteriorly in the middle. Immediately 

 beyond the condyle appears a depression, on either side of which are 

 seen the precondyloid foramina for the transmission of the hypoglossal 

 nerves ; they are extremely' small, and open anteriorly. External to 

 these, lying in the same line transversely, is seen a group of usually 

 three foramina for the passage of the eighth nerve and the internal jugu- 

 lar vein. The lateral terminations of the occipital, the paroccipital pro- 

 cesses, are large, thin, pointed forwards, and on a lower level than the 

 rest of the bone, forming a large part of the floor of the tympanic cavity. 

 The semi-elliptical contour of the cranium, regarding it from a basal 

 view, is well carried out laterally by the mng-like and attenuated mas- 

 toids. They contribute largely to the formation of the walls of the tym- 

 panic cavity internally, and externally assist in some degree towards 

 completing the temporal fossae. These fossae are deep ; commencing 

 posteriorly on either side at the external borders of the depressions 

 already mentioned that bound the cerebellar x)rominence laterally, they 

 take a course upwards and outwards, terminating at a foramen that lies 

 just within the posterior periphery of the orbit, which foramen allows 

 the passage of the tendon of the temporal muscle. From the upper 

 boundary of the temporal fossas to where the frontals suddenly abut 

 against and even overhang, to some extent, the nasals, the external and 

 superior surface of the skull is of a pearly whiteness and very smooth in 

 the dry skeleton, presenting not a trace of the sutures between the bones 

 that go to form it, the frontals and parietals. This surface is divided 

 by a well-marked furrow, that extends in the median line between the 



