DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 155 
and the keel is grooved; the hinder two-fifths is in two forks, that run back and gently 
bend in between the basal beam (p.e.) and the postero-inferior recesses of the basal 
labyrinth. Here the winged divisions of the vomer serve to bind together the right 
and left tracts of the nasal floor, under the fore half of the middle turbinal region. 
‘This second upper binding together of the right and left face, making the skull doubly 
desmognathous, is assisted by a pair of oblong splints, the posterior paired vomers 
(v’.), which run from this great opening between the inferior and middle turbinals up 
to the oval hinder recess, right and left, of the presphencid. When the middle 
turbinal is well ossified in its lower as well as in its front part, then the three 
vomerine bones and the ossified floor and turbinals all become an indistinguishable 
tract of bone. This preparation is only figured up to the hind margin of the foramina 
ovalia (V*.) ; the optic, ophthalmic, and maxillary nerves (IL, V'., V*.) are figured, 
emerging from the optic foramina and sphenoidal fissures. The rest of the skull in 
this aspect is seen in the entire palatal view (Plate 20, fig. 1). 
Viceral arches of the Fifth Stage. 
The lower jaw (Plate 22, fig. 8) is now almost complete, but there is a little cartilage 
still left on the coronoid and angular processes. The latter (ay p.) is somewhat 
incurved. The fore part of Mecket’s cartilage and the basimandibular rod (mk., 
bmn.) ave still present, but the freed part of the bar is now ossified as a large spatulate 
processes gracilis (pr.g.), much larger than the manubrium (mp.).* The incus 
and stapes (7., s¢.) are now ossified, and are seen in situ, so also is the annulus («.ty) ; 
all these parts are shown from their inner fuce. 
The hyoid arch (Plate 22, fig. 9) has acquired all its bony centres, but the thick, 
erescentic hypohyals (h.hy.) are only ossified in their middle, and the upper cerato- 
hyal (c.hy.) only half-way up. The epihyal (Plate 20, fig. 1, and Plate 22, fig. 9, e.hy.) 
is ossified in the upper part directly from the bony substance of the opisthotie (op.) ; 
it has no separate proper centre (“ tympanohyal”), nor splint (“stylohyal”). 
Sixth Stage of Skull of Evinaceus europzeus ; young ; two-thirds grown. 
These large young are profitable for study, because, although well ossified for the 
most part, nearly all the sutures are very visible; a vertical section at this stage 
(Plate 20, fig. 4) is very instructive. 
In the fore part we see the rounded form of the snout, and the complete septum 
that has grown forwards from the proper septum nasi between the alinasal folds (al.n.). 
The whole partition wall, from the front of the snout to the fore-edge of the pre- 
sphenoid (p.s.), is two-thirds the length of the whole craniofacial axis. The highest, 
or hinder part, is ossified as the perpendicular ethmoid ( p.e.), and this reaches forwards 
for nearly a third of the length of the wall behind the snout. The bone runs further 
forwards above than below; laterally, behind, it is now confluent with the partially 
* The letters of reference should have been m/ 
MDCCCLXXXV. pe 
