22 6 FISHES CHAP. 



cartilages, they form a complete concave roof for the protrusible 

 spotit-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 cranii, 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 prem axilla 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 



' 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. 



