DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 129 
Likewise we see the three posterior sphenoidal osseous centres (al.s., b.s.), the three 
lower centres of the occipital arch (¢.0., b.0.), and, also, the two upper centres that form 
this subsequently single keystone piece or proper supraoccipital (s.o.)—as distinct 
from the interparietal which may coalesce with it. 
Over the auditory capsule the pterotic band (s.a.c.) is notched ; behind that notch, 
looking downward, we partly see the recess under the arch of the anterior canal for 
the ‘ flocculus cerebelli.” 
Visceral arches of First Stage. 
The mandibular arch of this stage (Plate 22, figs. 1, 2, m/., mk., b.mn.) is a remark- 
able structure, being composed of both an outer and an inner “ramus.” Part of the 
epibranchial element of the first arch will afterwards be described as the pterygoid 
cartilage, the only remnant of the huge overgrowth of this part seen in Selachians. 
The quadrate region is greatly masked in this case—that of the Mammal—for the 
orbital process or “ pedicle” is suppressed, and the small pterygoid remnant is far off, 
forwards, whilst the small reduced quadrate segment is thrown into the same line, 
along the under face, as the large, well-developed lower segment, the ceratobranchial 
element, or articulo-Meckelian rod. Moreover, comparing the upper segment (incus 
or quadratum) with that of a Newt we see that its attachment to the skull is simply 
by the “otic process,” close in front of the ampulla of the horizontal semicircular 
canal. Also this segment turns inwards below, behind its articulation with the free 
mandible, and has a narrow-necked, dilated process to articulate with the dilated 
remnant of the extrastapedial (the flat face on the head of the stapes). 
The articular region of the endocranial mandible is already a “ malleus,” and 
already the “ processus gracilis” is there as a delicate ectosteal plate under the neck 
of the malleus, where the long, rounded part, or MecKeEt’s cartilage, begins. The 
special development of the Mammalian skull, with its much-tilted, auditory capsule, 
and closely-fitting lower face, makes that hinge vertica/, which is horizontal in all the 
Oviparous types. A short process on the inside of the cartilaginous malleus answers 
to the “posterior angular process ” of the mandible of the Bird ; whilst the ‘ internal 
angular process” of the Bird, especially that of the Fowl-tribe (Gallinacew), is largely 
developed, but tethered to the centre of a radiating plate, and acted upon by one 
special mandibular adductor muscle, now called the “tensor tympani.” The rim of this 
radiating plate is becoming cartilaginous, and is part of the jointed, cartilaginous 
lining of the ear-passage (meatus), but is ready to become bone, even now ; it does 
quickly become the “annulus tympanicus.” Here, at present, the mandible, proper, 
or MecKEL’s cartilage, becomes more and more solid, forwards, and somewhat flattened ; 
it is well-nigh equal, at this stage, to that of a Selachian. It becomes alate or dilated 
near the end, and then, in a peculiarly Mammalian manner, unites with its fellow, and 
the two are finished off in front by a long basimandibular spike (figs. 1, 2, ).2n.). 
But this huge main rod already lies in a groove on the inside of a rapidly ossifying 
MDCCCLXXXV. s 
