SKELETON. 75 
' fossa, and find the malleus and incus in the columella. For this they 
have no evidence except comparisons with certain theriomorph reptiles. 
The literature, which is extensive, should be consulted for details. 
The Skull in the Different Classes. 
CYCLOSTOMES have only the cartilage skull, and this can be homologized 
only in part with that of other vertebrates; indeed the skulls of the two groups of 
cyclostomes are not easily compared. The peculiarities are in part due to the 
development of a suctorial mouth with its necessary framework. The chondro- 
cranium of the Ammoccete stage of Petromyzon 
is readily understood. Parachordals, otic capsules 
and trabeculae (fig. 72) are normal, but a pair of 
ventral horns are problematical. Their position 
in front of and below the otic capsule renders 
doubtful the interpretation of hyoid or quadrate 
sometimes given them. 
The adult Petromyzon has a typical brain 
trough, roofed by a slender synotic tectum and 
fibrous tissue and closed in front by the unpaired 
nasal capsule, bound to the rest by fibrous tissue. 
The cranium is continued forward by a large plate 
(mesethmoid?) lying dorsal to the mouth, this 
part being roofed by two ‘dorsal cartilages,’ the 
anterior articulating with the annular cartilage Hie 4 — Barly -Gidadios 
supporting the mouth. A subocular bar extends cranium of Ammoceete stage 
‘forward from each otic region and an elongate of Pelromyzon, after Schneider. 
3 i h, hyoid; nc, notochord; oc, otic 
lingual cartilage extends from the mouth back capsule; ¢r, trabecule. : 
to the gill region. Several other elements occur, 
the names and positions of which may be seen from the figures. 
The myxinoid skull, the development of which is unknown, is readily inter- 
preted so far as basilar plate, trabecule and otic capsules are concerned. The large 
nasal capsule is continued forward by a latticed framework for the naso-hypo- 
physial canal and a bar (pterygoquadrate) joins the trabecula of either side and in 
front is continued in a cornual cartilage. The lingual cartilage is enormous (is it 
the lower jaw as has been suggested ?), is divided into three segments and bears a 
dental plate with teeth at its tip. There are cartilage axes to the tentacles around 
the mouth. 
The branchial skeleton of the lampreys consists of a gill basket of continuous 
cartilage with fenestre for the gills and above and below them as well. It cannot 
be homologized with the branchial skeleton of other vertebrates as it lies imme- 
diately beneath the skin and is lateral to gill pouches and aortic arches. It is more 
easily compared to the extrabranchials (p. 65) of. the elasmobranchs. The 
branchial apparatus of the myxinoids is reduced, consisting of two true gill arches, 
in front of which is another arch, usually interpreted as a hyoid. 
