288 MR. W. K. PARKER ON THE OSTEOLOGY OF BALAZNICEPS REX. 
it represents the ‘‘ conjoined bases of the orbito-sphenoids in mammals:” we quite 
coincide with Dr. Hallman and Professor Huxley that its supposed representative in 
the Carp is the orbito-sphenoid ; but not with Agassiz (with whom Professor Owen once 
agreed in this matter) that it isa cranial ethmoid. It does not agree with that hinder 
part of the connate orbito-sphenoids in the Carp which props up, and therefore lies 
below as well as in front of the ali-sphenoids (see Professor Huxley’s figure of the section 
of the Carp’s skull, Croon. Lect. p.24)'. This is not really a digression (for the crania 
of the Vertebrata mutually explain each other); we will therefore give one word more 
about this elegant little bone. In the Sea-bream (Pagellus centrodontus) the structure 
of a typical fish can be well seen. Looking at the floor of this creature’s skull, three 
remarkable bridges of bone are seen along the medial line: the hindermost of these is 
formed by the meeting of the ex-occipitals above their centrum; the middle bridge is 
formed in the same way by the petrosals, the basi-sphenoid lying at a great distance 
below the cranial floor ; whilst the first bridge is formed by a single bone (this ‘ ento- 
sphenoid ’), as though it were the serial homologue of these two pairs of bones behind. 
If this most anterior bridge belong to the same category as the petrosals and lateral 
occipitals, it should not directly precede the former were it the orbito-sphenoid ; and it 
is not a lateral element ; for perched up above and between it and the petrosals are the 
ali-sphenoids—pretty constant in fishes, but always high up, being feeble ossifications 
of the antero-lateral parts of the primordial skull, and lying in the midst of the still 
unossified cartilage that projects forwards on each side from the boundary of the great 
fontanelle. In the Reptilia proper the orbito-sphenoid is seldom present as a distinct 
bone ; for, although the roof-bones of the pre-sphenoidal sclerotome are largely developed 
as the ‘ principal frontals,’ the neurapophyses and centrum are arrested. In the Batra- 
chia they break out again, the neurapophyses forming the ring, and the centrum (pre- 
sphenoid) the septum, of the ‘os en ceinture’ (see Goodsir, op. cit. p. 157). 
Pre-sphenoid. (Pl. LXV. fig. 1, psp & eth.) 
The pre-sphenoid of the Baleniceps is large and well-developed: no part of it can 
be seen from above, the roof-bones being very perfect ; but it projects a line or so in 
front of them, just touching its serial homologue the ethmoid. All trace of the union 
of the orbital processes of the sphenoido-frontals with the pre-sphenoid is lost; but 
the groove for the olfactory nerve, and a descending process on each side marking 
the limits of the nasal fossze (which are continued backwards for four lines on each side 
of the pre-sphenoid at its upper part) show where we are. This descending process is 
just behind the ‘hinge,’ half an inch mesiad of the orbital margin. A strong fascia 
connects it with another smaller projection on the side of the thick carinate anterior 
part of the pre-sphenoid, half an inch below the widely-dilated part which props up the 
* The so-called ‘ ento-sphenoid’ is properly an ‘ orbito-pre-sphenoid’ in such fishes as the Carp and Salmon ; 
but we class the little Y-shaped bone of the Percoids with the ‘rostrum’ of birds and the higher reptiles. 
