PELONEUSTES. 3 1 



part of the posterior vertical semicircular canal (p.v.c.) ; this passage communicates 

 with the large rounded fossa (a.) situated on the antero-internal face of the opisthotic 

 and lodging the ampulla of the canal ; this same cavity also communicates with the 

 channel of the horizontal semicircular canal (h.e.), the opening of which is in the 

 middle of the facet for the prootic. The inner face of the exoccipital is concave from 

 above downwards ; it is perforated by two foramina, the upper (f.) perhaps for a blood- 

 vessel, the lower for the hypoglossal nerve (XII.) ; this latter opens externally on the 

 posterior wall of the outer opening of the jugular foramen (jug.). This foramen is 



exoA 



bdcf bocf. 



Bones of the auditory region of the skull of Peloneustes philarchus (E. 3803, | nat. size) : A, prootic, 

 inner face ; B, prootic, outer face ; C, supraoccipital, anterior face ; D, exoccipital-opisthotic, inner 

 face ; E, exoccipital-opisthotic, anterior face. 



a., cavities for ampulla; a.v.c, channel for anterior vertical semicircular canal ; boc.f., facet for basioccipital 

 borne by exoccipital ; boc.f.', facet for basioccipital borne by opisthotic ; exo.op.f., facet for exoccipital- 

 opisthotic ; /., ? vascular foramen ; h.c, channel for horizontal semicircular canal ; jug., jugular foramen ; 

 par./}., paroccipital process ; p.f., facet for parietals ; pro./., facet for prootic ; p.v.c, channel for 

 posterior vertical semicircular canal; q.f., quadrate facet on paroccipital process; s., remnants of 

 suture between the exoccipital and opisthotic ; soe.f., facet for supraoccipital : XII, foramen for the 

 hypoglossal nerve. 



enclosed between the exoccipital behind and the opisthotic in front ; it is a large oval 

 opening which no doubt transmitted the vagus nerve together with other nerves and 

 vessels ; externally it opens beneath the base of the paroccipital process and. as men- 

 tioned above, receives the outer opening of the hypoglossal canal. On the inner face 

 the posterior edge of the opisthotic forms a prominent oblique ridge, overhanging the 



