[21] THE OSTEOLOGY OF AMIA CALVA. 767 



becomes dubious. To render this horaological comparison safe, we must 

 have the positire proof, now missing, that this branch penetrates from 

 the outer side of the skull to the inner in this series of fishes. The 

 further distribution of the facial nerve after it quits the brain case is 

 of no further interest in the present connection. 



The orbital region is very definitely marked off. Its posterior bound- 

 ary has already been alluded to ; anteriorly the antorbital process, with 

 its ossification, the prefrontal, divides it from the nasal region. In Amia 

 the orbits are tolerably flat and oval depressions^ separated from one 

 another in the median plane by an antero-projecting process of the 

 Icavum cranii (Figs. 9 and 10); there is not a trace present in Amia of a 

 bony or membranous interorbital septum, as we find in so many of 

 the Teleostei. 



The roof of the orbit is formed only to a limited extent by a carti- 

 laginous, laterally-projecting ledge of the primoidal cranium,, which 

 one may consider as the last remnant of a cartilaginous orbital roof 

 (Figs. 2 and 3), the greater part of this roof being furnished by the 

 frontal bone. An orbital base is indicated by a feebly developed, wing- 

 like ledge projecting from the basis cranii, which is in contact with the 

 parasphenoid beneath (Figs. 9 and 10). 



The anterior third of the wall of the orbit is entirely cartilaginous,^^ 

 while the jjosterior two-thirds are in part occupied by two ossifications. 

 There is a large foramen found in the posterior p&vt of the orbit, bounded 

 above, behind, and in front by serrate edges of bone and below by car- 

 tilage, which opens into the brain case (Plate I, Figs. 2 and 3, Op.). 



Posteriorly through this opening passes the optic and several other 

 nerves out of the cranium, and through it the muscles of the eye reach 

 the skull ; anteriorly it is closed by a strong fibrous membrane. In 

 many of the skulls of the Selachians one can see a fenestration of the 

 lateral wall of the cranium, which is an extension of the foramen opticus, 

 and it does not appear very improbable to me that the foramen I have 

 just described in Amia is to be regarded as such a foramen opticus, 

 much enlarged. At the boundary line between the labyrinth and 

 orbital regions the cartilaginous base of the cranium is further pierced 

 by a small foramen, which is covered by the parasphenoid, and which is 

 only disclosed by removing that bone (Plate I, Fig. 3, fJi.). This foramen 

 in its position corresponds to the hypoplysis — to be described further 

 on — and is to be compared in many bony fishes to that lengthened cleft 

 at the base of the fossa for the muscles of the eye, which is closed by 

 the parasphenoid. 



The alisphmoid, constituting as it does the posterior ossification of 

 the orbital region, is of a circular form, with a section cut from it below 



^'In a large specimen of Amia I saw the lateral, as well as the side toward the 

 median plane — facing towards the cavum cranii — of this anterior orbital Cartilage 

 covered by a thin superficial layer of a brownish color, which at first sight looked Irke 

 a very thin lamella of bone. A microscopical examination showed here that we had 

 to deal with a calcification of the superficial layer of cartilage. 



