200 
MR, W. K. PARKER ON THE STRUCTURE AND 
The space between the true cranial sac and the trabecular undergirders, which in the 
Lamprey (Muller, op. tit. pi. 4. figs. 1, 3, 9, 10) permits the passage backwards of the 
azygous nasal pouch (fig. 1, k"), reappears by a sort of retrograde metamorphosis in the 
Mammal ; here the pouch, or rather air-cavity, is common to two symmetrical nasal sacs, 
and not to an azygous sac as in the Lamprey. In the Aye- Aye (Owen, Trans. Zool. Soc. 
1863, pi. 20. fig. 6, q) this air-sac completely divides the proper cranial floor from its 
trabecular addition, up to the very point at which the trabeculse became confluent with 
the investing mass in the early embryo. Another return to embryonic conditions is 
well seen in the complete separation of the “pars petrosa” from the sphenoidal and 
occipital regions of the skull ; this is most exquisitely seen in Scotophilus pipistrellus. 
In Mammalia the second visceral arch (mandibular) never undergoes segmentation 
above; the upper part of the head of the malleus answers to the “ quadrate” (Huxley, 
“ Malleus and Incus,” p. 402, fig. 6, M), the rest of the head to the articular region, 
and the processus gracilis to the Meckelian bar*. The “articular” region gives off a 
manubrium similar to the “opercular process” of the “ hyo-mandibular” (“ extrastape- 
dial”) ; and this serves the same purpose, namely to stretch the tympanic membrane. 
The third bar (second “ postoral”) becomes very much subdivided ; its segments are 
well shown in Professor Huxley’s figure. The upper part of the first postoral cleft is 
converted into the joint-cavity between the “malleus and incus;” the rest forms the 
tympanic cavity and Eustachian tube. The “ suprastapedial ” region is here represented 
by the head and “short crus” of the incus, the “ medio-stapedial” by the “long crus,” 
the “ interstapedial ” by the “os orbiculare ;” there is no extrastapedial. 
The remainder of the hyo-mandibular region or upper half of the primary bar is a 
small “ infrastapedial ” (“ infrahyomandibular”) rod («), which runs parallel with the 
tendon of the “stapedius” muscle. The stylo-hyal” (st.h) is separated from this little 
rod, from the lower end of which it was segmented at an earlier period (Mr. Huxley’s 
figure is from a foetus five months old) ; these segments are now separated by nearly all 
the length of the belly of the “ stapedius.” 
The decurved and transverse position of the infrastapedial is evidently due to the up- 
ward movement of the freed “ stylo-hyal ;” most of the tissue of the bent and broken 
part of this visceral arch has been converted into the tendon and belly of the “ stape- 
dius.” The head of the stylo-hyal, overgrowing the bent portion above it, has ascended 
to the “ opisthotic” region, and has completely coalesced with it as in the Common 
Toad ; this is a coalescence of two cartilaginous tracts totally unrelated morphologically. 
In the Mole (Talpct europcea) the part of the malleus articulating with the incus is 
half severed from the rest, forming an upper quadrate lobe ; the “ short crus” of the 
incus is very feeble. A similar well-marked quadrate lobe is seen in the Shrew ( Sorex 
tetragonurus ); and it has a still smaller “short crus” to the incus; the “orbiculare” is, 
relatively, nearly as large as in the Frog. 
* The fore part of the Meckelian rod is ossified separately as the “inferior intermaxillary rudiment” (see 
Callender, Philosophical Transactions, 1869, Plate NTII. fig. 6, a, p. 170) ; there the Batrachian structure is 
repeated; the same “ mento-meckelian ” ossicle may be seen in old Sturgeons. 
