190 



ON THE STRUCTURE OF THE SKULL. 



superadded membrane bones, but no cartilage bones. D. The 

 osseous cranium. 



The three first-mentioned kinds of skull are met with only 

 among fishes; Amphibia, Beptilia, Aves, and Mammalia in- 

 variably possessing* a larger or smaller number of cranial bones 

 developed in cartilage. 



A. The membranous cranium. 



The only animal, at present known, which comes under 

 this category is that singular fish, the lowest of all Vertebrata, 

 Amphioxus lanceolatus (Fig. 74). The notoehord (Ch), sur- 



Fig. 74. 



Fig. 74. — Skull of Amphioxus lanceolatus (after Quatrefages). — a, Position of olfactory (?) 

 sac ; h, optic nerves ; c, fifth (?) pair ; d, spinal nerves ; c, representatives of neural 

 spines ; /, g, oral skeleton ; Ch, notoehord ; My, spinal chord, or myelon. 



rounded by a merely membranous sheath, extends very nearly 

 to the anterior pointed extremity of the body. The myelon, or 

 spinal chord {My), occupies the ordinary position above it, in a 

 canal formed by upward processes of the membranous sheath, 

 and gives off the spinal nerves, d d, on each side. Quadrate 

 masses of somewhat denser tissue, e e, seem faintly to represent 

 neural spines. Just above the anterior boundary of the mouth, 



