The oral cirri of Siluroids and llie oriffin of the head in Vertebrates. 383 



articulation is a very flat one, showing that there can be little move- 

 ment of the hyomandibular 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 continued 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 supra- 

 cranial fontanelle, and somewhat further forward, there is a large 

 open s])ace in the cartilaginous wall. This is tilled up by thin bone 

 and does not transmit nerves or blood vessels. Below this a carti- 

 laginous 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 wall, tilled up however by thin bone. 



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

 canal (iV. orh. c.) can be seen. It transmits a vein and the Ophthal- 

 micus superficialis, VII. The nerve runs in a foramen of the carti- 

 lage 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 re- 

 placed by bone. 



The preorbital process is well developed, but not of great verti- 

 cal 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 cartilaginous septum between the olfactory nerves and lobes of 

 each side, and from this septum a roof extends to the orbital wall 

 and preorbital process, so that the olfactory nerves and lobes lie in 

 long tunnels of cartilage , except where this is replaced irregularly, 

 above and below, by thin bone. In the model these replacements 

 appear as asymmetrical open spaces on each side of the rostral region. 



The anterior narrower portion of the brain case, that is, from 

 the epii)hysial bar forwards, is drawn out to a remarkable length, as 

 indeed are all the structures of the anterior part of the head. The 

 hyomandiliular cartilage (H.M.) is triangular in form, the lower angle 

 being jtroduced into a strong process, which bears the operculum 

 without the intennediation of an opercular cartilage. An extensive 

 l)ut thin block of cartilage (Qu) remains in the symplectic and qua- 

 drate region, and sends inwards and forwards a short pterygoid process. 



25* 



