Yol. 69.] CKANIAL ELEMENTS OP DAPEDWS GRANULATUS. 221 



sphenoid is seen in the recent Beryx, and, it has been suggested, 

 has something to do with the depth of the body ; Zeus faber, 

 however, is an example of a deeper-bodied fish with a practically 

 straight parasphenoid. 



The basipterygoid processes are produced upwards and backwards 

 across the anterior edges of the alisphenoids, and meet the orbito- 

 sphenoids behind their inferior projection. Between these pro- 

 cesses is the entrance to the orbit of the basicranial canal, which 

 lies within the posterior portion of the parasphenoid and contained 

 the rectus externus muscles of the eye. 



In Amia calva the canal extends for only half the distance 

 between the orbit and the rear of the cranium, ending in a cul de 

 sac, this being the same in Salmo salar and in most of the Teleostei. 

 In Goregonus ivarlmanni, as exemplified by Yogt in his ' Embryo- 

 logie des Salmon es,' the canal extends to the extreme rear of the 

 skull, and this occurs also in Dapedius. The truncated section of 

 the parasphenoid shows a distinctly ossified oval chamber, which lay 

 against the under surface of the notochord as it entered a deep 

 conical excavation apparent in the basioccipital. 



In Amia the canal gives entrance to the orbit to the oculo- 

 motorius, trochlearis, and ophthalmic branches of the trigeminal 

 nerves ; but in Salmo salar it is pierced only by the nervus abducens 

 which supplies the rectus muscles. This statement probably holds 

 good of Dapedius also, the other nerves being provided with an 

 ample exit in a large opening immediately above, situated between 

 the orbito-sphenoids, which probably affords a passage to the 

 main branches of the trigeminal and facial nerves as well. 



The opisthotics are stout and prominent bones inclined upwards 

 anteriorly at a considerable angle to the basal elements of the skull, 

 this angle corresponding with the inclination of the fore part of the 

 parasphenoid. Posteriorly they join the exoccipitals, and anteriorly 

 the postfrontals and alisphenoids. They are excavated below to 

 receive the suspensorium, and in front there are depressions which 

 gave attachment to the suborbitals. 



The opisthotics on each side of the head form the floor of the 

 temporal fossa, which, triangular behind, expands anteriorly into a 

 circular pit. The fossa is roofed by the pterotic, and in the 

 specimen at the British Museum (Natural History) is covered at 

 the side by the squamosal. 



A large trifoliated aperture is shown in the bony interorbital 

 septum in conjunction with the orbitosphenoids, which, meeting at 

 their bases, form the roof of the entrance to the basicranial canal, 

 and again meeting in an upper and forward projection join the 

 ethmoids. 



At the superior posterior angle of the orbit on each side the 

 orbitosphenoids are pierced by an irregularly-shaped foramen for 

 the passage of the optic nerve ; anteriorly, in conjunction with the 

 ethmoids, they form a passage for the olfactory nerves, each of 

 which, enclosed for the greater part of the orbit, enters it at the 

 opening shown as i, fig. 1 (p. 219), and leaves again by the inner 



