71] THE SKULL OF AM I URUS— KINDRED 71 



bone is overlapped by the suprasphenoid and the ventral, by the parasphenoid; 

 suprasphenoid and parasphenoid bones fusing at the posterior margin of the 

 bone. Anteriorly the orbitosphenoid thins out and passes gradually into the 

 cartilage of the intemasal septum. The middle part of the bone has entirely 

 ossified, but the persistence of cartilage beneath the external lamellae of the 

 lateral parts of the bone shows that ossification does not extend uniformly 

 through all its parts. The frontal overlaps the dorsal margin of the bone in 

 the cavum wall and in line with the epiphysial bar region the alisphenoid and 

 orbitosphenoid meet above the optic foramen. Considerable cartilage remains 

 in both of these bones in this region, but between this point and the anterior 

 end, the part of the orbitosphenoid which forms the wall of the cavum is well 

 ossified. Below the optic foramina the orbitosphenoid-suprasphenoid inter- 

 digitation continues from one side to the other within the cavum, marking the 

 posterior limit of the orbitosphenoid as a lining bone of the cavum cranii 

 (Fig. 7). 



The developmental relations of the orbitosphenoid have been given for the 

 32 mm. stage (p. 52). The perichondrial ossifications in the wall of the cranium 

 which now form an integral part of the bone were then just beginning. From 

 the description just above, it is evident that there is considerable cartilage yet 

 remaining within these osseous lamellae. The ledge on the external surface 

 of the bone, between the orbital and optic foramina, is developed from con- 

 nective tissue surrounding the ventral end of the alisphenoid cartilage and the 

 trabecula cranii, and is intimately connected with the perichondrial ossifica- 

 tions of these cartilages (Fig. 3). The stout median part of the bone is devel- 

 oped from perichondrial ossifications which have fused with each other across 

 the anterior end of the fenestra hypophyseos (Fig. 4). In the younger stages 

 the cerebral hemispheres lie immediately dorsal to the orbitosphenoid region, 

 but in the adult they are quite far posterior to it, and the olfactory tract sur- 

 rounded on each side by a gelatinous mass extends above it. The anterior 

 end of the jugular vein enters the cranium through the orbital foramen and 

 proceeds posteriorly along the dorsal surface of that part of the orbitosphenoid 

 which forms the wall of the cavum cranii. 



The earlier descriptions were confusing because of the absence of this bone 

 in some of the lower teleosts, especially since it is absent in the perch, Cuvier's 

 type of fish cranium. The present identity of the bone with the ala orbitales 

 of mammalian anatomy is derived from the work of Kallmann (1837). He 

 recognized Cuvier's error in naming the petrosal, the ala magna and hence 

 confusing the homologies of the more anterior parts. Since the bone next in 

 front of the petrosal in mammals was the ala magna of the sphenoid, and since 

 this bone in the carp had relations corresponding to it, the name was applied. 

 The bone in front of this was then compared to the 'ala parva' and found to 

 correspond, hence it was named. Agassiz (1842) and Stannius (1853) did 

 not adopt this homology, but named the bone the os ethmoideum. The 



