SEA-FISHERIES LABORATORY. 159 



side being the larger and more densely calcified. They are 

 bounded by the frontal, alisphenoid, prootic, pterotic and 

 parietal. 



Prootic or Petrosal (Pr.O., figs. 2, 3). — A stout bone 

 containing a quantity of cartilage. It is perforated by the 

 large canal or foramen jugulare (f.jug.), which transmits 

 the internal jugular vein, the ophthalmic artery and the 

 truncus hyomandibularis nervi facialis. It also forms 

 the postero-lateral wall of the trigemino-facial foramen 

 (f.tr.fa) and the external wall of the carotid foramen 

 (f.car.), transmitting the internal carotid artery. Further 

 it forms the internal and lower half of the hyomandibular 

 cup (Hm.F. 1 ), and its part in forming a false floor to the 

 cranial cavity by processes of bone and cartilage has been 

 already mentioned. The prootic has two conical depres- 

 sions on its cerebral surface, the ventral one being much 

 the larger. It is bounded by the parasphenoid, 

 alisphenoid, sphenotic, pterotic and basioccipital. 



Epiotic (Ejj.O., figs. 1, 3, 4). — A dense structure 

 largely cartilaginous, but having a thin outer shell of 

 bone, prolonged into the somewhat prominent epiotic 

 process (Ep.P., fig. 4). The cerebral surface bears two or 

 three deep conical pits with a thin bony lining, one being 

 much larger than the others. The epiotic provides the 

 remainder of the cartilage for the occipital cross already 

 mentioned. It is bounded by the supraoccipital, parietal, 

 pterotic and exoccipital. 



Pterotic (Pt.O., figs. 1, 2, 3). — Forms the greater part 

 of the parotic process (Pa. P., fig. 4). It is more densely 

 calcified than the epiotic, and its cerebral surface bears 

 three deep conical pits, one being partially subdivided 

 into two. Laterally it bears an imperfect oval bony facet 

 for the posterior condyle of the hyomandibular (Hm.F. 2 , 

 figs. 2, 3, and cp. fig. 5). The left facet is appreciably 



