— 41 — 



The alisphenoid of Amia and teleosts is thus, in principle, an inverted Y-shaped bone, the 

 anterior arm of the inverted Y resting on the lateral edge of the actual or potential basisphenoid, 

 and the posterior leg resting on the lateral and morphologically posterior wall of the orbital opening 

 of the rayodome, the ascending process of the parasphenoid there coming into supporting relations 

 with it. The bone thus straddles the orbital opening of the myodome, and through the passage between 

 its legs, in Amia, the oculomotorius, trochlearis, and profundus nerves enter the orbit; the passage 

 not, however, representing the fused foramina of those nerves (Allis, '97b), their true foramina being 

 where they respectively pierce the membrane which, in Amia, forms the entire lateral bounding wall 

 of this part of the cranial cavity. All three of the nerves, in Amia, certainly lie anterior to the para- 

 sphenoid leg of the alisphenoid; and the profundus certainly lies posterior to the basisphenoid leg of 

 that bone. What the relations of the trochlearis andoculomotorius are to this latter leg is not evident, 

 for there is nothing in the continuous membrane to in any way indicate the alisphenoid region. In 

 teleosts, where, as in Scorpaena, this membrane of Amia may be replaced by bone, the relations of 

 these two nerves to the basisphenoid leg of the alisphenoid are also not evident; for although, in my 

 descriptions of Scomber, I stated that, in that fish, the trochlearis issued along the antero-mesial 

 edge of the alisphenoid, a reconsideration of the figures makes it evident that the so-designated edge 

 of the bone is, in reality, part of its ventral edge. These two nerves, in teleosts, thus both seem to 

 have been pushed downward, by the growing ventral edge of the alisphenoid, and to lie, in the adult, 

 simplv ventral to that bone, without positively evident anterior or posterior relations to it. 



In Gasterosteus, where, according to Swinnerton, the alisphenoid is wanting, a dorsal Pro- 

 longation of the ascending process of the parasphenoid has invaded the region of the parasphenoid 

 leg of the alisphenoid, and there has come into sutural contact with the closely adjacent ventral 

 edge of the ventral process of the frontal. This condition of the parasphenoid is also found in Cottus 

 octodecimospinosus. and will be fully described when describing that fish. It is apparently also found 

 in the Barbidae and Cobitiidae, and in Homaloptera (Sagemehl '91, p. 564). 



S P H E N T I C. 



The sphenotic (postfrontal, postorbital ossification) is an irregulär bone that forms the summit 

 of the postorbital process of the skull. It forms part of the inner as well as part of the outer surface 

 of the brain case. Its outer surface has lateral, dorsal and anterior regions, separated by sharp angles. 

 The anterior surface forms part of the hind wall of the orbit. On it there is a relatively large recess, 

 beneath a thin Hange of bone, and from this recess the oticus canal leads upward laterally and back- 

 ward in a curved course, traversing the bone and issuing on its dorsal surface near its hind end. It 

 transmits the ramus oticus lateralis, accompanied by both communis and general cutaneous fibers, 

 and is the homologue of the similar canal described by me in Scomber. On the lateral surface of the 

 bone there is a large articular facet for the anterior head of the hyomandibular, the ventral portion 

 of the facet being formed by the proötic. Immediately dorso-anterior to this facet there is a roughened 

 surface which gives origin to the levator arcus palatini, and immediately dorso-posterior to the 

 facet there is a depression, open posteriorly, which forms the anterior part of the dilatator fossa. 

 The dorsal surface of the bone, which is flat, gives support to the postfrontal bone and also to the 

 anterior edge of the pterotic and the lateral edge of the posterior portion of the frontal. The internal 

 surface of the bone is relatively small, and presents, as in Scomber, two deep recesses separated by 

 a thin and nearly vertical partition of bone which, projecting backward and mesially into the cranial 



Zoologica. Heft 57. 6 



