70 
MR, W. K. PARKER OK THE STRUCTURE AND 
the Lamprey, but it has already gone beyond the suctorial type in having both paro- 
steal and ectosteal plates; it is profitable also for comparison with that of the larvae of 
Dactylethra and Pipa. 
The free growth of cartilage, here, gives us a character in this type which I have 
not found in others, but which Gotte describes in Bombinator, namely, that the noto¬ 
chord (nc.) is ensheathed in an azygous, tubular cartilage, as in the Selachians.* 
Hence the parachordals are very narrow bands between this tube and the ear-sacs ; 
and very short also; for the trabecular apices embrace the fore end of the notochord. 
The cartilage, right and left, has formed a perfect arch above, and the flat roof runs 
up to the post-orbital region (Plate 2, figs. 1 and 3, s.o.). 
Behind (Plate 2, fig. 3), the occipital condyles are forming; they look directly back¬ 
wards, and the bony matter (e.o.) runs into them. The auditory capsules were not 
relatively large at first, but they are so largely developed outwards now as to seem 
of unusual size, being winged above and below. 
By the study of the growth of the skull in many types I have satisfied myself that 
the upper wing (“ tegmen tympani ”) has not the same morphological import as the 
lower, or floor of the tympanium, both of which are enormously developed. 
The basal plate, formed by the trabeculae and parachordals, grows round the base of 
the ovoidal ear-sacs, and appears outside, especially in front: hence this copious growth 
of cartilage, forming elegant lower wings to the ear-masses, and serving as a shelving 
tympanic floor (fty.). This undergrowth showing itself outside the ear-sacs, appears 
in many forms ; it is a manifest striving of the basal plate to meet the arches that 
belong to it, this junction being impeded by the huge sense-capsules impacted in at 
this part. 
But, as in the “ Aglossa,” the “tegmen tympani” (t.ty.) is a superficial growth of 
cartilage applying itself to the capsule, in the margin of which is imbedded the hori¬ 
zontal canal; this roof-plate is very large, still larger than the floor (Plate 2, figs. 1-3). 
This tegmen is very thick as well as wide ; it is widest in front, and there it over¬ 
laps, and is confluent with, a large process from the elbow of the suspensorium ( ot.p .). 
Here the tegmen cranii, and the tegmen tympani, of each side, remain soft, but the 
labyrinth is almost entirely enclosed in a generalised occipito-petrosal mass ( e.o.,pr.o .). 
This is an undistinguishing spread of bony matter over and under these regions, a 
division into “periotic” and “ex-occipital” being attempted later in the growth of 
the head. 
It is very solid bone, and leaving the azygous cartilage untouched below, and a 
similar breadth of roof above, it builds a side wall to the foramen magnum, forming a 
solid rough kind of masonry, and enclosing most of the “canals” and part of the 
“ sacculus.” 
The cartilaginous stapes (Plate 2, fig. 3 ; Plate 10, fig. 5, st .) lies in its fenestra just 
under the middle of the “tegmen,” and above the deep shelving floor {fty.) 
* The outer sheath of the notochord is extremely thin in most of the Batrachia. 
