DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 165 
crus of the incus (s.c.7.) as the morphological equivalent of the “ otic process” of the 
quadrate of a Bird, Reptile, or Amphibian, and remembering that the “ orbital 
process” is suppressed in this Mammalian suspensorium, we see by this figure that 
the incus is well in front of the auditory capsule. Indeed, it is as definitely in front as 
in the embryo of any of the Sauropsida, in some of which the otic process, ulti- 
mately, runs backwards to be articulated with the fore edge of the occipital arch, 
beyond the auditory capsule, altogether. 
The body of the incus (7.), as well as its two crura, is already of the normal shape, 
and so also is the malleal portion of the primary mandible, with its bulbous head, and 
its large, forwardly-turned manubrium (ml., mb.). The large, terete, sigmoid Meckelian 
rod (mk.), after uniting with its fellow of the opposite side, runs into a basimandibular 
spike (b.mn.). 
Here, normally for a Mammalian hyoid arch, but not for a branchial arch, as such, 
there are three segments below the flat, rounded epihyal (e.y.), which is not united 
with its own uppermost segment, but with the auditory capsule. Nor is it united 
with the next segment, or upper ceratohyal (c.hy.), by cartilage, for that next lower 
piece is fibrous above. 
The two next segments are the straight lower ceratohyal (c.iy.), and the curved 
hypohyal (h.hy.); this is the stoutest of all the segments. The common rudiment of 
the basihyal, basibranchial, and first hypobranchiais (b.h.br., t.hy.), is a stout, well- 
formed U-shaped piece, with the front edge of which the two hypohyals are articu- 
lated. Over the tympanic cleft, the tympanic annulus (a.ty.) is seen, formed of a 
crescent of tender bone round the membrana tympani (77.ty.). 
Fourth Stage.-—Skull of embryo of Mole; 3 and $ of an inch long. 
Dissection of the chondrocranium of an embryo Mole, 3 inch long. 
The basal view of this skull (Plate 25, fig. 2) shows the distinctness of the olfactory 
and auditory sense-capsules from the cranium proper, and the upper view (fig. 3) 
displays the large amount of cartilaginous “tegmen,” in spite of the cruciform 
fontanelle. The general outline of the skull is pyriform, the narrow nasal end being 
the stalk ; this is dilated, in front, over the inferior external nostrils (e.7.), and again 
where the alinasal region (a/.n.) ends, opening below at the beginning of the aliseptal 
region (al.sp.), where the inferior turbinals (7.tb.) are given off. At its middle the 
nasal labyrinth swells out, suddenly, so that it is itself pyriform, and ends in front 
of the orbitosphenoid (0.s.) in a large and somewhat bilobate cushion ; the right and 
left masses are separated by the perpendicular ethmoid (p.e.) which passes into the 
septum nasi (s.v.). The long alinasal region is closed below, except at the sides, 
in front ; and the hinder fourth of the main labyrinth is also perfectly floored with 
cartilage. The open space between these two floored regions is largely filled up by 
the huge recurrent lobes (rc.c.) which support JACcoBson’s organs; these tongue-like 
tracts are three-fourths the length of the long open space, and are themselves supported 
