SKELETON OF EEGALECUS AEGENTEUS. 9 



mass of cartilage, having a straight lower edge embraced by the vomer (vo), and a 

 curved anterior and dorsal border continuous posteriorly with the tegmen cranii. 



At about the middle of the roof of the skull the tegmen cranii is perforated by two 

 median apertures ; the anterior of these (figs. 8, 11, and 12, /) probably transmits the 

 delicate crura of the olfactory lobes j of the functions of the other I am quite ignorant ; 

 very possibly it serves for the passage of an artery. 



On each side of the base of the rostrum, and piercing the base of the antorbital 

 process, is a large oval fofamen (figs. 8 and 9, V 1 ), which doubtless transmits the orbito- 

 nasal nerve. 



On the lateral surface of the hinder, or auditory region of the cranium is a sigmoid 

 depression in the cartilage (fig. 7, h.m 1 ) ; with this, the large hyomandibular is articu- 

 lated, as shown in fig. 6 (h.m). 



A longitudinal vertical section of the cranium shows that the hinder third of the 

 basis cranii (fig. 11, b.cr.) is raised into a sort of step ; I was unable to ascertain 

 whether this had any definite relation to the form and disposition of the brain. The 

 same section shows that the form of the cranial cavity is largely determined, not 

 merely by the bones and cartilages which form its proper Avails, but by the immensely 

 thick periosteum with which these latter are lined. In fig. 11, the cranium is shown 

 with the periosteum (coloured blue) still in place ; in fig. 12, in which it has been 

 removed, the form of the cavity is seen to be quite changed. 



The ossifications of the cranium are twenty-seven in number, of which twenty-two 

 are paired and five median. 



The basioccipital (b.o) is a vertical plate of bone constituting the posterior and dorsal 

 portion of the subcranial crest. Above, it becomes much thickened and forms the 

 medio-ventral facet of the occipital condyle. Although it enters into the formation of 

 the basis cranii, it does not actually bound the cranial cavity, since it is covered by the 

 exoccipitals (e.o) and opisthotics (op.o). It is the best-ossified bone in the skull, and 

 shows no trace of its original cartilaginous groundwork. It articulates above with the 

 exoccipitals and opisthotics, below with the parasphenoid (pa.s), and in front with the 

 descending processes of the opisthothics (op.o *). 



The exoccipitals (e.o) are two irregular bones bounding the occipital foramen late- 

 rally, and, by their union with one another in the middle ventral line, forming also its 

 lower boundary. Each furnishes one of the dorso-lateral facets of the occipital condyle 

 (o.c J ), and is perforated by the vagus foramen (X), The posterior face of the exoccipital 

 is nearly flat, its inner face is irregular (see fig. 12), and is excavated by a deep pit 

 leading into a tunnel for the posterior semicircular canal (cf. fig. 11). The united ex- 

 occipitals form the upper part of the elevated posterior third of the basis cranii, its 

 lower part being formed by the basioccipital and opisthotic, with which bones the ex- 

 occipitals therefore articulate below. Externally, each articulates with the pterotic 

 (pt.o), and dorsally with the epiotic (ep.o). 



vou xn. — part i. No. 2. — February, 1885, c 



