DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
13 
the septum nasi ( s.n .); the vomer ( v .) under it, and the palatine plates of the maxil- 
laries and palatines ( mx.,pa .) are shown as forming the hard floor to the shallow 
naso-palatine canal. The “fault” in the skull wall, where the small alisphenoid is 
thrust out, is partly filled in externally by the squamosal (sq.). 
First Stage (continued).— Visceral arches of Tatusia hybrida. 
This stage, although more advanced than some I have been able to work out in 
certain Insectivora, is yet very valuable and instructive ; the two mandibles—the 
outer and the inner —are now well developed, but the peculiar Mammalian transforma¬ 
tion of the inner mandible has not yet taken place. 
The superficial ramus (Plate 2, fig. 3, d.) is full four times the bulk of the primary 
articulo-Meckelian rod (mk), and reaches back words nearly to the thick articular 
region. 
A row of small alveoli is seen in front of the coronoid process ( cr.p .), which is 
peculiarly long and slender, and forms an acute angle with the condyloid part (cd.p.), 
a semi-oval mass of cartilage. The tip of the coronoid process is also cartilaginous, 
and so is the edge of the feebly developed angular process ( ag.p .), 
Meckel’s cartilage—the inner or primary mandible—is a very uniform terete rod, 
lying all along the inner face of the ramus, near its lower edge ; it follows the sinuosi¬ 
ties of the ramus up to the “ mentum,” where it is confluent with its fellow (fig. 2, mk.). 
At this confluence there is developed a tongue-shaped non-segmented rudiment of a 
basimandibular element ( h.mn ,); this is more distinct in the Insectivora than in the 
Armadillos. There is no appearance, at this stage, of a distinct splenial, or of a 
distinct coronoid, bone inside the ramus, in this type. There is, however, here, as 
everywhere in the Mammalia, a very large tract of cartilage on which the hinder 
half of the dentary is formed, and out of which the new articular facet, that on the 
lower edge of the squamosal, the “ glenoid ” facet, is also developed. 
This subcutaneous slab of cartilage is manifestly the homologue of the lower labial 
of cartilaginous Fishes, as much as the bone itself corresponds with the antero- 
external splint of Meckel’s cartilage in Ganoids and Teleostei, the Amphibia and 
the Sauropsida—the well-known “ dentary.” 
A preparation of the whole auditory and articular region of the skull shows many 
things in their proper relations (fig. 4). Part of the squamosal (sq.), with its glenoid 
facet (gif), is figured, and the condylar (cd.p.) and angular part of the ramus or lower 
jaw, with its solidly cartilaginous hinder margin. 
Meckel’s cartilage, large and solid, is seen running along a groove on the inner face 
of the ramus, and in the other direction shows itself as a large pars articulare, with 
its sinuous condyloid face for articulation with a remarkable form of quadrate—now, 
evidently, the “incus,” before ossification. 
As in the Fowl and many other Birds, there is a long “ internal angular process ” to 
