— 48 — 



The membrane in which they develop is, in my 45 mm specimens, a thick layer of coarse fibrous 

 tissue which extends from the opticus to the abducens foramina, passing on either side of the pituitary 

 fossa. This membrane is continuous, on either side, with the side wall of the skull, the ventral and 

 larger portion of that wall here being also of membrane. This latter membrane is attached dorsally 

 to the ventral edge of a cartilaginous process of the auditory capsule which forms the dorso-lateral 

 corner and the dorsal portion of the side wall of this part of the skull. This process is called by 

 Swinnerton ('02), in his descriptions of Gasterosteus, the postorbital process of the auditory capsule; 

 but, as already stated, supraorbital process would seem a better term, for the process extends antero- 

 mesially from the postorbital process of the skull along the dorsal edge of the posterior portion of 

 the orbit. The membranous side wall of the skull ventral to this process is, in my 45 mm specimens, 

 undergoing ossification to form parts of the proötic and alisphenoid bones, and the former bone, 

 or that part of the membrane that will ossify as part of it, is perforated, immediately ventral to the 

 ventral edge of the cartilage, by two foramina. One of these foramina is a large opening which trans- 

 mits the united trigeminus and lateralis trigemini nerves and also the encephalic branch of the jugular 

 vein, the other transmitting the facialis and lateralis facialis nerves. Ventro-anterior to these two 

 foramina, but still posterior to the slightly developed basisphenoid bone, the membrane is pierced 

 by both the oculomotorius and truncus ciliaris profundi; these two perforations lying relatively 

 close together, the one for the ciliaris profundi slightly dorso-posterior to the one for the 

 oculomotorius. 



In the region ventral to the profundus foramen the membranous cranial wall is connected 

 by a bridge of dense, coarse, fibrous tissue, with the dorsal end of the ascending process of the para- 

 sphenoid, that part of the latter process that is cut in sections passing through this region, forming 

 the lateral wall of the orbital opening of the myodome and lying anterior to the cartilage that repres- 

 ents the ventral portion of the proötic. The dorsal end of the process of the parasphenoid here lies 

 at a relatively considerable distance from the membranous side wall of the cranial cavity, and from 

 its anterior edge, and continuous with the bridge of fibrous tissue that spans the space between it 

 and the membranous cranial wall, a strong line of tissue runs dorsally, and, separating into two parts, 

 has its attachment to the cranial wall, one part dorsal and the other ventral to the Ophthalmie nerves. 

 This line of tissue lies wholly anterior to the truncus maxillo-mandibularis trigemini, and represents, 

 in part, the two little process-like ridges on the external surface of the alisphenoid of the adult, and, 

 in part, the fibrous or connective tissues that extend from those little processes to the dorsal end of 

 the ascending process of the parasphenoid. The line of tissue that has its insertion dorsal, and hence 

 morphologically postero-lateral, to the Ophthalmie nerves, represents the postero-lateral one of the 

 two processes of the adult, the lme that has its insertion ventral, and hence morphologically antero- 

 mesial to the nerves, representing the antero-mesial process: the two bands of tissue together repres- 

 enting the parasphenoid leg of the alisphenoid, here pierced by the Ophthalmie nerves, as the pedicle 

 of the alisphenoid is in Amia. The basisphenoid leg of the alisphenoid is represented in an undefineel 

 portion of this part of the cranial wall, that wall being of membrane in its ventral portion, but already 

 ossified in its dorsal portion. The bridge of fibrous tissue that spans the space between these two 

 legs of the alisphenoid, represents that part of the orbital surface of the proötic of the adult that is 

 oecupied by the internal jugular groove. 



Slightly posterior to the oculomotorius and profundus foramina the lateral edge of the bridge 

 of fibrous tissue above referred to sends a line of tissue downward internal to, and parallel to the 



