Brooks — The Osteology and Arthrology of the Saddoek. 171 



The sphenotio continues the parotic ridge forwards. It unites 

 hehind with the pterotic ; above and behind, with the parietal ; 

 above and in front, with the frontal ; and below and in front, with 

 the alisphenoid. The sphenotiois often called " post-frontal"; but 

 the post- frontal is a membrane bone, while the sphenotio is pre- 

 formed in cartilage.^ 



The alisphenoid is the smallest of the cranial bones, and is of 

 an irregularly quadrilateral form. The upper edge unites with 

 the frontal ; the posterior^ with the sphenotic ; the lower with the 

 prootic. The anterior or free edge shows a groove, in which 

 the nasal branch of the trigeminal nerve runs up to reach the 

 frontal bone. To this edge the membranous interorbital septum 

 is attached. 



The prootic presents on its anterior border a deep notch, 

 through which the branches of the trigeminal and facial nerves 

 pass out of the cranium. It unites below with its fellow of the 

 opposite side, and below this with the parasphenoid, the three bones 

 bounding a deep pit, which is open anteriorly, and gives origin to 

 the recti muscles of the orbit. It articulates above with the ali- 

 sphenoid, sphenotic, and pterotic, and behind with the opisthotic. It 

 covers the anterior portion of the vestibule and the anterior semi- 

 circular canal. Behind it articulates with the basioccipital in the 

 manner described below. 



The basioccipital presents posteriorly a concave articular sur- 

 face for the atlas, which differs from the articulation of an ordinary 

 vertebra only in being bevelled slightly above. Just above this it 

 articulates with the exoccipitals, and in front and below this arti- 

 culation it unites with the opisthotics. Mesially below, a tapering 

 wedge of bone stretches forwards, and fits in between two similarly 

 shaped processes of the parasphehoid, which, passing backwards, 

 fit into grooves on each side of the basioccipital wedge. Viewed 

 from above, the bone shows a pair of deep excavations, separated 

 by a median ridge, the anterior truncated extremity of which 

 articulates with both prootics, which also articulate with the ante- 

 rior edge of the bone external to this. These sutures are not seen 



^ A post-frontal and sphenotic may co-exist, as in Amia (Macalister, Morphology 

 of Vertebrate Animals, p. 72). 



- More properly, postero-esternal. 



SCIEN. I-ROC. R.D.S. — ^\-0L. IV. PT. IV. it 



