THE DIPNOI. 



169 



one continuous cartilaginoi^s mass, into tlie base of which 

 the notochord penetrates, terminating in a point behind 

 the pituitary fossa. 



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

 occipital, supra- occipital, basisphenoid, or presphenoid; 

 and there are only two such ossifications, which represent 

 the ex-occipitals {E. 0.) in the side-waUs of the cranium. 

 A large parasphenoid {x) underlies the base of the skull. 

 Upon its roof a great single bone {A), answering to the 

 parietals and frontals, extends from the occipital to the 

 ethmoidal regions. In front of this are two nasal bones (C). 



Fig. .5.3. 



Fig. 53. — Longitudinal and vertical section of the sliull of Lepidosiren. 

 The cartilage is dotted ; the membranous and bony constituents are 

 shaded with lines. A, B, C, D, E, Hy, as in the preceding figure ; 

 X, .r.the parasphenoid ; P.S', cartilaginous presphenoidal region ; ch, 

 notochord ; Au, situation of auditory chamber ; 1, 2, first and second 

 vertebrae; //., F., VIII., exits of optic, trigeminal, and vagus 

 nerves ; a, quadrato-mandibular articulation. 



There is no alisphenoid, but the fronto-parietal and para- 

 sphenoid send processes towards one 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 

 skuU remains of tolerably even diameter throughout. In 

 front of the exit of the optic nerves, however, it is longi- 

 tudinally divided by a membranous septum. 



The ethmovomerine cartilage is continued to the anterior 

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

 vomer. 



