THE SKULLS OF FISHES AND AMPHIBIA. 



209 



Behind the auditory chambers the cartilage is almost ex- 

 cluded from the walls of the skull by two lateral ossifications 

 of its substance — the ex-occipitals (E.G.). As in the Amphibia, 

 there is no ossified supra- or basi-occipital. The rest of the lateral 

 parietes of the skull would be devoid of bony walls were it not 

 that the parasphenoid (x) and the great bone (A), which roofs 

 in the whole length of the skull, and answers to the frontals 

 and parietals, send upwards and downwards, respectively, lateral 

 processes, which unite together, and so replace the alisphenoid 

 (Fig. 85). The ethmo- vomerine cartilage {Eth. Vo.) bears, supe- 

 riorly, the nasal bones (C), and inferiorly it carries teeth (E). 

 A long flat bone, pointed posteriorly (B, B), is attached to the 

 hinder edge of the nasals, and roofs over the orbit and temporal 

 fossa. 



Fig. 85. 



Fig. 85. — Longitudinal and vertical section of the Skull of Lepidosiren. The cartilage is 

 dotted ; the membranous and bony constituents are shaded with lines. A, B, C, D, 

 as in the preceding figure ; x, x, the parasphenoid ; I, 2, the first and second ver- 

 tebral arches ; Ch, the notochord ; Au, the situation of the auditory organ. 



The notochord, which forms the chief axis of the spinal 

 column of this fish, is continued into the base of the skull, and 

 ends in a point about the level of the exit of the trigeminal 

 nerves (V). There is neither basi-occipital nor basi-sphenoid, 

 and the presphenoid is represented only by the cartilaginous 

 floor at (P./S"). The pterygopalatine apparatus is represented, 

 on each side, by the great dentigerous curved plate (D), which 

 is applied to the inner surface of the cartilaginous sub-ocular 

 process, abuts against the parasphenoid by its inner edge, and 

 descends to the inner side of the articular condyle for the 

 mandible (a). The hyoidean arch {Hy) is attached to the 

 middle of the posterior and inferior edge of the sub-ocular 



p 



