146 



THE ANATOMY OF YERTEBRATED ANIMALS. 



The spinal column consists of a tliick notocliord, invested 

 by a cartilaginous sheath, without aiiy 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-rajs support the median fin. The skull, the palato- 

 quadrate, and suspensorial apparatus, form, as in CldinCBra^ 

 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 

 (x) 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 



B A ^t 



Fjg. 53. — Longitudinal and vertical section of the skull of Lepidosiren. The cartilage is 

 dotted ; the membranous and bony constituents are shaded with lines. A^ B^ (7, 2), E^ 

 Hy, as in the preceding figure ; a?, a?, the parasphenoid ; P, /S^, cartilaginous presphenoi- 

 dal region ; ch^ notochord ; Au, situation of auditory chamber ; 1, 2, first and second 

 vertebrae; //., F., VIIL, exits of optic, trigeminal, and vagus nerves; a, quadi-ato- 

 mandibular 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 [D) extends from 

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



