DR. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
523 
in one (badly preserved) example a wide foramen at the inside and near the angle of the 
mouth leads into this pouch ; during life it was probably closed by the mucous mem- 
brane of the mouth. 3. The lower labial cartilage (see also Plate XXXV. fig. 3, la), 
forming a broad trenchant fringe round the extremity of the mandible. 
The part of the cartilage which bears the two trenchant front teeth I consider to be 
the vomer (Plate XXXIV. fig. 3, v). It is very similar in form to the vomer of many 
Teleosteous fishes, and has two free lateral edges, whilst behind it is suturally connected 
with the pterygo-palatines ; it is entirely cartilaginous, and passes into the sphenoid car- 
tilage above the symphysis of the pterygo-palatines. The corresponding part in Lepido- 
siren has been regarded by Professor Owen* as the intermaxillary, and by Professor 
Bischoff, Peters, and others as belonging to the ethmoid or nasal bone, Professor 
Bischoff and Peters considering the cartilaginous space behind the palatal arch to be 
the vomer. From the fact that the vomer occupies nearly always the same position in 
the skull of fishes, and from the frequency of its being armed with teeth, I conclude 
that this part of the Ceratodns- skull is the vomer. A comparison with the skull of 
Menopomci will also much assist in determining it as such. 
Towards the upper surface of the skull the vomerine cartilage passes into the ethmoid 
or nasal portion f, which is easily distinguished by its double perforation by the olfactory 
nerves (Plate XXXV. fig. 1, h', and fig. 2, oh) ; its upper surface has a thick osseous 
layer (Plate XXXIV. fig. 1, &, and Plate XXXV. b), which appears as a distinct trian- 
gular bone without median suture, the apex being directed forwards, whilst its base 
forms a serrated suture with the frontals and sclero-parietal ; anteriorly it passes into 
the rostral cartilage. 
The frontal bones (Plate XXXIV. fig. 1, c, and Plate XXXV. fig. 1, c) are entirely 
separate from each other ; their anterior portion, which forms part of the border of the 
nasal cavity, and also the upper edge of the orbit, is very solid and closely connected 
with the underlying cartilage ; it emits, in front of the orbit, a strong, broad, concave 
process (c J ) for union with, and support of, the upper molar. The posterior part is an 
oblong, thin lamina, extending to the hindmost edge of the skull, to which it is fastened 
by fibrous tissue ; this part covers the temporal muscle, and is attached by squamous 
sutures to the tympanic lamina and sclero-parietal. It is the same bone which appears 
as a free process on each side of the head of Lepidosiren ; Professor Owen determined 
it as postfrontal (Trans. Linn. Soc. xviii. p. 334, pi. 23. fig. 5, m), adding that it repre- 
sents the postorbital and supratemporal bones in Ganocephala (Anat. Vertebr. i. p. 85). 
Hyrtl does not much differ by designating this bone as superciliary, which, although a 
distinct bone in most fishes, is in reality nothing but a detached part of the frontal. 
* In referring to Professor Owen’s account of the osteology of Prctopterns, not only his first memoir in the 
Linn. Trans, vol. xviii. is to he consulted, hut also the figure (No. 41) in Anat. of Yertehr. vol. i., where some 
inaccuracies of the former account are corrected. 
t So designated in Lepidosiren by Peteks and Muller ; Bischoff considers it to he the intermaxillary ; and 
Owen also does not distinguish it from the “ tooth-hearing cartilage.” 
