M6 THE ANATOMY OF VERTEBRATED ANIMALS. 



The spinal column consists of a thick notochord, invested 

 by a cartilaginous sheath, \vithout any osseous or cartilagi- 

 nous vertebral centra. The proximal ends of ossified neural 

 arches, of ribs, and, in the caudal region, of inferior arches, are 

 imbedded in the sheath of the notochord. 



Fin-rays support the median fin. The skull, the palato- 

 quadrate, and suspensorial apparatus, form, as in CMrrKxra, 

 one continuous cartilaginous mass, into the base of which the 

 notochord penetrates, terminating in a point behind the pitu- 

 itary fossa. 



No cartilage bone is developed in the place of the basi-occi- 

 pital, supra-occipital basisphenoid, or presphenoid ; and there 

 are only two such ossifications, which represent the ex-occipitals 

 (E. 0.) in the side-walls of the cranium. A large parasphenoid 

 (a;) underlies the base of the skull. Upon its roof a great single 

 bone (^), answering to the parietals and frontals, extends 

 from the occipital to the ethmoidal regions. In front of this 

 are two nasal bones { C). There is no alisphenoid, but the 

 fronto-parietal and parasphenoid send processes toward one 



Fib. 58. — Lon^tudinal and vertical section of the skull of L&pidosiren, The cartilage U 

 dotted ; the membranous and bony constituents are shaded with lines. A^ £, C, -D, S, 

 Hijy as in the preceding figure ; c, ?■, the parasphenoid; P, iS^, cartila^ons presphenoi- 

 dal region ; cft, notochord ; Aii^ situation of auditory chamber ; 1, 3, first and second 

 vertebrffi; /Z, 7"., 77//., exits of optic, trigeminal, and vagus nerves; a, quadi-ato- 

 mandlbuiar articulation. 



another, which unite in front of the exit of the third division 

 of the fifth nerve. There is no interorbital septum, and the 

 cavity of the skull remains of tolerably even diameter through- 

 out. In front of the exit of the optic nerves, however, it is 

 longitudinally divided by a membranous septum. 



The ethmovomerine cartilage is continued to the anterior 

 extremity of the skull. It bears teeth, but no distinct vomer, 



A great palato-pterygoid osseous arch {V) extends from 

 the middle line along the upper and the under surface of the 



