112 PROF. T. JEFFEET PARKER ON THE 



from the skull alone, the trigeminal probably passes out through the foramen A, that 

 is, leaves the skull in front of the bone in question. 



I do not think, however, that the place of each of the nerves can possibly be held as 

 absolutely determining the homologies of bones with which they are related. In many 

 Mammals the first division of the fifth passes out altogether in front of the alisphenoid, 

 and the second and third divisions through foramina in that bone. The spinal nerves 

 of Vertebrates, too, sometimes perforate the neui-al arches instead of passing through 

 the intervertebral foramina. 



The bones under discussion are the hindermost paired ossifications in the trabecular 

 or preauditory region of the skull ; I think, therefore, they must be considered as ali- 

 sphenoids, but that, by a curious variation, each of them has joined with the corre- 

 sponding opisthotic behind the trigeminal foramen. Or the case may be better put 

 thus: — the nerves are formed long before the ossifications appear; in the present case, 

 the prootic is so small as not to extend so far forwards as the trigeminal foramen, and 

 the ophisthotic and alisphenoid meeting in front of it, that is, between it and the 

 foramen, have caused the latter to be situated in front of the alisphenoid instead of 

 between it and the prootic. 



The oilitosjilienoids (o.s). — In front of each alisphenoid is a thin flat bone which forms 

 the anterior and ventral region of the supraorbital plate and is united (? suturally i) 

 with its fellow in the middle ventral line, forming a false floor to the brain-case in front. 

 It articulates above with the orbital plate of the frontal, behind with the alisphenoid, 

 and in front with the mesethmoid (ni.eth). It is an ectostosis, and evidently represents 

 the orbitosphenoid. 



The mesethmoid {m.eth). — This is apparently a partly ectosteal, partly endosteal 

 bone, occurring at the junction of the tegmen cranii with the prenasal rostrum ; it 

 extends around the orbitonasal foramina (fig. 8, V^), and though the cartilage to its 

 ventral surface (figs. 11 and 12), where it articulates behind with the orbitosphenoids. 

 The mesethmoid contains a cavity (figs. 11 and 12, m.n.c), which forms a vertical 

 cleft between the tegmen cranii and the prenasal cartilage, and evidently represents the 

 mesonasal cavity of the Salmon ^. It is, however, much smaller than in the latter fish, 

 and is lined and roofed by the mesethmoid, whereas in the Salmon it is mainly sur- 

 rounded by cartilage, and is covered in above by a parostosis, the supraethmoid or 

 dermo-ethmoid. 



The ectethmoids or parethnoids {p.eth) are paired ectosteal ossifications of the ant- 

 orbital region, articulating above with the frontals. They are largely subcutaneous 

 bones, being covered only by the thin silvery epiderm {cf. fig. 1). 



The remaining ossifications of the cranium are parostoses, or membrane-bones. 



' See note on p. 9. 



' W. K. Parker " On the Structure and Development of the Skull in the Salmon," Phil. Trans, vol. 163, 1873, 

 p. 108 ; Parker and Bettany, ' Morphology of the Skull,' p. 72. 



