35] THE SKULL OF AM I URUS— KINDRED 35 



The anterior part of the alisphenoid cartilage, as we have noted, is unossi- 

 fied at this stage (Figs. 3,4,) and is probably converted into the orbitosphenoid 

 bone of the adult which appears in this region (Figs. 16, 20). The perichon- 

 drial ossification between the optic and trigeminal foramina (Fig. 4) is the 

 beginning of part of the alisphenoid bone of the adult (Fig. 20). 



In the younger stage the internal carotid artery enters the cranium internal 

 to the trabecula (Fig. 2 ca), but now it has a different relation. This blood 

 vessel enters a rete mirabile, lateral and posterior to the optic foramen, and a 

 branch to the internal part of the cranium enters through the posterior part of 

 the optic foramen. The external carotid enters the cranium as before, between 

 the branches of the trigeminal nerve. 



The ramus oticus of the facial nerve (Fig. 3 rot. VII) issues through the 

 dorsal posterior margin of the alisphenoid cartilage just anterior to the otic 

 capsule, and proceeds posteriorly within the lateral line canal ossification on the 

 sphenotic bone to innervate a canal sense organ. 



Speaking in a general way of the orbitosphenoid development throughout 

 the whole vertebrate series, Parker and Bettany say that it is the result of the 

 ossification of the anterior part of the lateral cranial wall, and may be either 

 anterior to or penetrated by the optic nerve. It also arises from paired centers, 

 although in the adult it may be unpaired. It is described by these authors in 

 Salmo as an 'ectosteal' (comparable to my term perichondrial) lamina in the 

 anterior part of each side wall of the cranium. Concerning the development 

 of the same bone in Salmo, Gaupp (1906) says that it is developed in the dorso- 

 anterior part of the interorbital septum and the ventral surface of tlie tegmen 

 cranii of this region, and, further, the olfactory nerve issues through it before 

 entering the orbit. Since an interorbital septum is absent from Amiurus, 

 comparison with the Salmonoid condition is not very easy and clear to the 

 casual observer. But if we can conceive of the cranium of Amiurus being 

 compressed instead of depressed, then the anterior ends of the alisphenoid 

 cartilages, where they unite with the ectethmoid process, would be pushed to- 

 gether, and the surface where they met would be comparable to the interorbital 

 septum. At this stage of Amiurus there is no ossification on the dorsal part of 

 the anterior end of the alisphenoid cartilage in the region of the anterior part 

 of the optic foramen, although the perichondrium shows signs of the beginning 

 of a perichondrial ossification, both on the outer and inner surfaces, which 

 extends down to the trabecula. The first traces of the orbitosphenoid occurs 

 in the Salmon at the 35 mm. stage. 



The ahsphenoid of the salmon is a later ossification than the orbitosphe- 

 noid, whereas in Amiurus it is quite well developed before any great develop- 

 ment of the orbitosphenoid. Speaking of the alisphenoid in Salmo, Gaupp 

 (1906) says: "Entsteht sehr spat (Salmo fario von 40 mm.) in Form von zwei 

 perichondralen Knochenlamellen, einer inneren und einer ausseren, auf der 

 knorpeligen Schadelseitenv/and vor der Ohrkapsul. Im Anschluss an den 



