16 ILLINOIS BIOLOGICAL MONOGRAPHS [16 



the membranous lateral semicircular canal and the ventral end of the anterior 

 enter the cavum cranii anterior to this septvmi laterale. The posterior end 

 of the membranous lateral semicircular canal and the ventral end of the poste- 

 rior, enter posterior to it. The septum semicircularis posterius lies in about the 

 same horizontal plane as the anterius and makes an angle of about 120 degrees 

 with the ventral surface of the roof of the capsule. The posterior end of this 

 septum is continued as a medial wall between the cavum of the posterior mem- 

 branous semicircular canal and the cavum cranii. The dorsal end of this canal 

 passes posteriorly above this septum. The fenestrae above both the anterior 

 and posterior septa are much smaller than the fenestrae anterior and posterior 

 to the lateral septum. 



After leaving the ganglionic mass of the faciahs, the ramus lateralis acces- 

 sorius of this nerve proceeds dorsally and curves around the anterior end of the 

 roof of the capsule and thence along the dorsal surface of the cartilage, above 

 the occipital arch to the body musculature. In all this distance the nerve is 

 unenclosed by cartilage, nor is there any indication of ossification around it. 

 It is accompanied by a branch of the internal jugular which descends and 

 fuses with the postcardinal vein in the region of the second neural arch. 



The hyomandibular cartilage articulates with the external surface of the 

 ventro-lateral wall of the anterior and lateral semicircular canals (Figs. 1,2). 

 This articulation extends from the ventro-anterior edge of the capsule just 

 above the posterior margin of the foramen for the facialis nerve, posteriorly in 

 an obUquely dorsal direction toward the lateral edge of the roof of the capsule. 

 The articular surface is very small in comparison with the longitudinal extent 

 of the capsule and at this stage there is no projecting shelf for this articulation. 

 In a nine day Ictalurus albidus chondrocranium (Ryder, 1886), the hyoman- 

 dibular articulation differs from that in Amiurus, the anterior end being more 

 dorsal than the posterior and, in addition, the surface is smaller and is over- 

 lapped by a process from the anterior margin of the capsule, suggestive of the 

 pterotic ridge of Polypterus (Budgett) in the same region. The hyomandibu- 

 lar articulation of other teleosts and some of the ganoids is in nearly the same 

 plane and region as m Amiurus, this scheme of articulation appearing to be 

 typical for these groups. 



The otic capsule of the Amiurus type is apparently derived from a primi- 

 tive condition represented by the Cyclostomes (Parker and Sewertzoff) and 

 found in the larval Acanthias as well. In these forms the otic capsules are 

 fused, ventrally, to the basal plate and the cavum of each communicates with 

 the cavum cranii by a large foramen through which the seventh and eighth 

 cranial nerves enter the capsule. The ninth and tenth nerves leave the crani- 

 um posterior to the capsule in the Cyclostomes, but are in the same relative 

 position in the larval Acanthias as in Amiurus. In Amiurus (p. 13) 

 these leave by separate foramina, the tenth between the otic capsule and the 

 occipital arch, the ninth a httle anterior. A description of the septa semicir- 



