226 FISHES CHAP. 
cartilages, they form a complete concave roof for the protrusible 
spout-like mouth. Palatine, mesopterygoid, and pterygoid bones 
invest, and in some measure replace these cartilages. In brief, 
the skull of the Chondrostei occupies an interesting intermediate 
position between the purely cartilaginous and mainly bony types. 
While retaining a well-developed and unossified primary cranium, 
it has acquired a complete secondary cranium of dermal bones. 
Equally notable is the condition of the jaws. Unique among 
the Teleostomi in possessing the typical Elasmobranch union of 
the palato-quadrate cartilages beneath the basis crani, the Chon- 
drostei are so far specialised that they have acquired certain of 
the membrane bones which constitute the secondary jaws of the 
more typical bony Fishes. 
As regards the general structure of the skull and the nature 
and disposition of its cartilage- and membrane-bones, the remain- 
ing living Teleostomi have much in common with- the Salmon. 
In all the skull is hyostylic, and, unlike the Chondrostei, each 
half of the primitive upper jaw remains distinct from its fellow, 
and is separately articulated in front with the lateral ethmoid 
of the same side by its palatine element. The palato-quadrate 
cartilage is always more or less completely replaced by bones 
similar to those of the Salmon, and although they often carry 
teeth, as a rule they do little more than constitute a rigid 
buttress for the fixation of the quadrate condyle for the lower 
jaw. The secondary upper jaw is nearly always well developed, 
and includes a premaxilla as well as a maxilla on each side. 
There are, however, certain features in each of the minor groups 
which are either distinctive or highly characteristic. 
In the surviving Crossopterygii (e.g. Polypterus ') the chondro- 
cranium is complete in the ethmoidal and post-orbital regions, 
except where it has been partially replaced by cartilage bones, but 
in the inter-orbital region the continuity of the roof is interrupted 
by a large fontanelle, which is only closed by the investing frontal 
bones (Fig. 132,C). There is also a large basi-cranial fontanelle 
in the sphenethmoid, closed, however, by the underlying para- 
sphenoid. A large “occipital” bone continuously ossifies in the 
occipital cartilage and completely surrounds the foramen magnum. 
Prootics and pterotics are absent, and the opisthotics seem to be 
1 Traquair, Journ. Anat. and Phys. v. 1871, p. 166; Bridge, Proc. Birm. Phil. 
Soc. vi. 1888, p. 118; Budgett, Trans. Zool. Soc. xvi. Pt. vii. 1902, p. 315. 
