the Gwlacanth Fish. 323 



formed by two strong rounded ridges, which subside into-the 

 flat outer surface anteriorly. 



Tiie ventral ridge overhangs a deep smooth groove, 

 running ah.)ng the side of the bone innnediately above the 

 upper edge of the paraspiienoid. This ritige is overhijjped 

 posteriorly by the anterior end of the prootic. 



Between the upper and lower ridges is another deep 

 rounded groove, which passes backward and is very nearly 

 converted into a foramen by a bowing outward of the anteri(n- 

 edge of the prootic ; the aperture so formed is regarded by 

 Stensio as the place of exit of the facial nerve, a view which 

 is undoubtedly correct. 



The upper ridge dorsally turns forward and outward 

 until it terminates in a rounded process, regarded by Stensio 

 -as a basipterygoid process, an interpretation which I once held, 

 but found it necessary to abandon. This process lies close 

 under, but is not in contact with, the skull-roof. From its 

 inner anterior face a thin lamina — Stensio's allspiienoid 

 — runs almost directly forward, terminating in a trnucaleil 

 extremity, which lies immediately below the hinder end (>£ 

 the frontal, but which is not fused with that bone, as it is 

 in Wimunia. This lamina is pirtially separated fron» the 

 " basipterygoid " by a deep, very narrow slit. 



The sphenoid is completed by a pair of very thin long 

 walls, wliich arise from a feebly ossified common base lying- 

 over the paraspiienoid and rise toward the roof of the skull. 

 The external surface of each of these plates is pierced by thiee 

 foramina. 



Otic Region. — The lateral walls of the hinder jiart of tiie 

 brain-case are largely formed by the great bones called 

 prootico-opisthotic by Stensio. I prefer to call these bones 

 prootic, because there is very little reason for believing that 

 they include a real opisthotic. 



Each consists of a body whose flat inner surface articulates 

 with the lower ridge of the sphenoid in front and with the 

 side of the basioccipital behind. 



From the outer surface of the body posteriorly a strong 

 ridge gradually rises as it is traced forward, until it turns 

 dorsally and then backward, sweeping round so as to form 

 a deep backwardly directed pocket, bounded mesially by the 

 body of the bone. The hinder end of this ridge comes into 

 contact and in adults fuses with a special descending flange 

 from that dermal bone, called by Stensio the su[)rattMnporo- 

 extrascapular. This descending flange of the su[)rateinporal 

 is continued forward by a deep slender ridge. 



21* 



