151 THE SKULL OF AM lURUS— KINDRED 15 



the trabeculae. There is a large fenestra hypophyseos in the chondrocranium 

 of the larval Polypterus (Budgett, 1907) which, from its ellipsoidal shape, is 

 comparable to that of Amiurus. In none of these, however, have the relation 

 of the internal carotid and trabecula been brought out. In all cases the nerves 

 of the cranial series, from the second to the seventh, issue from the cranium 

 above the trabeculae. 



In Salmo (Parker, 1872; Gaupp, 1906) the trabeculae unite immediately 

 anterior to the hypophysial region to form an elongated anteriorly extending 

 trabecula communis, on the dorsal surface of which the membranous interorbi- 

 tal septum rises. The relation of the trabecula to the nerves is typical, but 

 the trabeculae do not meet the ahsphenoid cartilages in the medial wall of the 

 orbit as in the Selachians, Ganoids, and Amiurus. Further comparisons of 

 the relations of the trabeculae in other groups than the fishes are made by 

 Parker and Bettany (1877), and Gaupp (1906). 



The otic region. The otic capsules at this stage are two large cartilaginous 

 masses forming the sides of the posterior region of the cavum cranii (Figs. 1,2). 

 Ventrally, they are fused with the basal plate of the posterior cranial floor, from 

 the posterior ends of the trabeculae to the occipital arch, and there is no gap 

 (basicapsular fenestra, Parker, in Salmo) between each capsule and the basal 

 plate. Posteriorly there is no line of division between the occipital arch and 

 the posterior boundary of the otic capsules. Anteriorly the cartilage sur- 

 rounding each auditory mass is confluent with the posterior end of the ah- 

 sphenoid cartilage. The dorsal medial margins, at this stage, do not meet above 

 the hind-brain to form a cartilaginous synotic tectimi as is found in other tele- 

 osts (Fig. 2). 



The vagus nerve passes obHquely between the otic capsule and the ventral 

 end of the occipital arch, latero-dorsal to the parachordal plate. The glosso- 

 pharyngeal nerve has a smaller and more anterior foramen in the floor of the 

 otic capsule and is separated from the foramen of the vagus by a small bar of 

 cartilage. 



The cavum of the otic capsule is fully open to the cavum cranii, except at 

 the extreme anterior end where there is a small medial wall, bounding the ante- 

 rior part of the anterior semicircular canal. The cavum within the capsule is 

 divided by three septa semicircularia into the cartilaginous labyrinth containing 

 the membranous semicircular canals. The septum semicircularis anterius, as in 

 the adult (Fig. 7), is a short bar of cartilage extending from the anterior wall of 

 the capsule posteriorly to the midventral surface of the roof, parallel to the long 

 axis of the body. From its dorsal connexion with the utriculus, the anterior 

 membranous semicircular canal passes above this septum into the anterior 

 part of the capsule. The septum semicircularis laterale is situated at right 

 angles to the anterior septum between the roof and the floor of the capsule^ 

 but nearer to the posterior than to the anterior septum. The anterior end of 



