276 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



other along nearly the whole of their length, entering into the forma- 

 tion of the anterior and posterior fontanelles, their articulation being- 

 only at a small surface about their middle point. The mucous 

 canals, which run in the pterotics and sphenotics continue their for- 

 ward course in these bones, which present many foramina or mucous 

 pores. On the upper surface one of these is especially noticeable, 

 situated on a level with the anterior extremity of the articular sur- 

 face on each side. Below, on the inner side of the vertical ridge, is 

 a small foramen which is for the exit of a small dorsal branch of the 

 trigeminus. On the outer side of the ridge are a varying number of 

 foramina, varying even on opposite sides of the same skull both as to 

 size and number. In front of these a groove runs forward to a fora- 

 men in the very front of the bone, opening into the nasal capsule 

 and giving passage to the opthcdmic branch of the fifth which exits 

 from the skull through the alisphenoid. The frontals articulate in- 

 ternally with the fellow of the opposite side ; below with the alis- 

 phenoids, orbitosphenoids and ectethmoids ; behind with the spheno- 

 tics and supraoccipitals. 



12. Orbitosphenoid, (PI. II. Fig. 2, Os.) 



A single bone forming the base and walls of the skull, the cavity 

 of which is contracted in this region, expanding both in front and 

 behind. It forms a passage or canal in which lie the olfactory nerves. 

 Immediately above the horizontal portion the bone is notched deeply 

 anteriorly and posteriorly. These notches are made foramina by the- 

 articulating bones. Through the anterior one a, vein passes, through 

 the posterior, the optic nerve. The orbitosphenoid articulates in 

 front with the ectethmoids and mesethmoids ; above with the frontals ;■ 

 behind with the alisphenoids and basisphenoid ; and below with the 

 paras'phenoid. 



13. Mesethmoid, (PI. II. Figs. 1 & 2, MEth.) 



Forms the anterior boundary of the skull, and enters into the for- 

 mation of the floor and the roof of the anterior portion which con- 

 tains the olfactory nerves. It is the median ossification of the eth- 

 moid cartilage of the young fish, and is one of the two bones in which 

 the ossification of the cartilage is not completed in the adult, the inner 

 surface of the bone being lined with it. In front it is notched, and 

 spreads out into two horn-like processes which articulate below with 



