208 ON THE STRUCTURE OF THE SKULL. 



The class Pisces presents us with a complete series of these 

 crania, from the least ossified forms, which possess only one or 

 two pairs of cartilage bones m the walls of the cranium, to the 



' Fig. 84. 



Mn y 



Fig. 84. — Lepidosiren. A, the parieto-frontal bone ; B, the supra-orbital ; C, the nasal ; 

 D, the palato-pterygoid ; E, the vomerine teeth ; E.O., the ex-occipital ; Mn, the 

 mandible ; Hy, the hyoid ; Br, the branchiostegal ray ; Op, the opercular plate ; x, 

 the parasphenoid ; y, the bone which gives attachment to the scapular arch ; Or, the 

 orbit ; An, the auditory chamber ; N, the nasal sac. 



completely ossified skulls of the Cyprinoids and Siluroids. And 

 again, just as among the preceding groups we found that the 

 Chimoeroids differed widely from the rest in having the sub-ocular 

 process, or arch of the skull, to which the mandible is attached, 

 formed of one piece of cartilage, which is continuous with, and 

 immoveable upon, the skull ; so, in this series, Lepidosiren is at 

 once distinguished from all the rest by a similar character. 



The skull of the Mudfish (Fig. 84) is composed of a frame- 

 work of cartilage, which sends down a broad triangular process, 

 on each side, to articulate with the mandible, and expands, 

 posteriorly and laterally, into chambers for the auditory organs. 

 Between these, the roof and the floor of the skull are both con- 

 stituted by cartilage ; but anterior to them, as far as the ex- 

 tremity of the parasphenoid (.*;), this tissue becomes very thin or 

 disappears (Fig. 85). In front of the anterior end of the para- 

 sphenoid, it makes its appearance again on both the roof ami 

 the floor of the cranial cavity, beyond which it is continued 

 as a thin lamella to the end of the snout. A fibrous septum 

 with a free concave, posterior margin, divides this region of the 

 cranium into two lateral chambers, one for each olfactory lobe. 



