ORAL CIRRI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 5 



not however extend up as a tegmen cranii, nor is it prolonged down- 

 wards to form the floor of the cranial cavity, and there is no appearance 

 of resorption of cartilage, to indicate that at an earlier ontogenetic 

 stage it extended further. A pterotic ridge is present, better developed 

 in its posterior than in its anterior part, and it bears the moderately long 

 articulation for the hyomandibular. The articulation is a very flat 

 one, showing that there can be little movement of the hyoman- 

 dibular on it. 



In the cranial floor, two blocks of cartilage occur, at the level of the 

 anterior semicircular canal. The cartilage of the auditory region is con- 

 tinued forwards as the wall of the orbital region, a bay in front of the 

 anterior semicircular canal indicating where the facial and trigeminal 

 nerves pass out. Slightly further forwards about the middle of the 

 orbital region, the epiphysial bar (Eph) passes across the supracranial 

 fontanelle, and somewhat further forward, there is a large open space in 

 the cartilaginous wall. This is filled up by thin bone and does not 

 transmit nerves or blood vessels. Below this a cartilaginous projection, 

 directed backwards at each side of the pituitary body, forms a partial 

 floor to the brain in this region. It leaves a deep notch below the 

 orbital Avail, filled up however by thin bone. 



Where the orbital joins the ethmoid region, a slit-like preorbital canal 

 (Pr. orb. c.) can be seen. It transmits a vein and the Ophthalmicus 

 superficialis, VII. The nerve runs in a foramen of the cartilage on 

 the right side of the drawing, the opening being seen from the front, 

 on the preorbital process. On the left side of the figure, a notch 

 corresponds to this foramen, the cartilage being partly replaced by 

 bone. 



The preorbital proces-i is well developed, but not of great vertical 

 extent. 



An extraordinarily well developed rostrum is present, triangular in 

 section in its anterior part, the upper angle truncated in the posterior 

 sections. This rostrum extends far back posteriorly, as a thick cartil- 

 aginous septum between the olfactory nerves and lobes of each side, 

 and from this septum a roof extends to the orbital wall and preorbital 

 process, S3 that the olfactory nerves and lobes lie in long tunnels of 

 cartilage, except where this is placed irregularly, above and below, by 

 thin bone. In the model these replacements appear as asymmetrical 

 open spaces on each side of the rostral region. 



