171 TEE SKULL OF AMIURUS— KINDRED 17.- 



cularia of the Cyclostomes is lacking, so a comparison with the inner surface 

 of the capsules cannot be made. However the structure of the ears in these 

 forms is so different from that of the Gnathostomes, that detailed comparisons 

 would have Uttle value here. 



In the larval Acanthias the fenestra of communication between the cavum 

 cranii and the cavum labyrinthii is as wide as it is in Amiurus, but a wall is 

 beginning to grow from the Une between otic capsule and basal plate, which 

 will eventually separate the two cavi. Sagemehl, from his comparative mor- 

 phological study of the crania of the teleosts, says that their condition in this 

 region is derived from the constant fenestration of the foramen for the auditory 

 nerve, rather than from the Cyclostome condition. The evidence given above 

 of the presence of a wide fenestra in the larval Acanthias is against his view 

 and in favor of the derivation of the condition in the teleosts from an ancestor 

 with a wide fenestra. 



The ninth and tenth nerves leave the cranium by separate foramina in 

 Acanthias just as they do in Amiurus, as stated above. The synotic tectimi 

 in Acanthias is formed very early by the growth of the median margins of the 

 otic capsules, a condition not reached by Amiurus until very late in the larval 

 period and then only for a short distance anteriorly. 



The relation of the capsules to the basal plate and to the cavum cranii of 

 Lepidosteus osseus (Parker, 1882), is much the same as in Amiurus except for 

 the large fenestra in the ventral floor of each capsule. There is no wall 

 between the cavum cranii and that part of the capsule containing the inner ear. 

 A detailed description of the septal relations is lacking. 



Part for part, the otic capsule of the larval Salmo as described by Gaupp 

 (1906) is nearer to the condition of Amiurus than any other that has as yet 

 been described. Except for the precocity in the growth of Amiurus they can 

 be said to be identical in all their relations, if the presence of the basicapsular 

 fenestra in the floor of the capsule be left out of consideration. There is the 

 same relation of cavum of the labyrinth to cavimi cranii and the same number 

 of septa semicircularia are present and have the same relation to the mem- 

 branous labyrinth in both forms. The relations of the ninth and tenth nerves 

 are homologous in both cases. Externally the hyomandibular articular sur- 

 face is about in the same region in both. The synotic tectimi of Salmo is very 

 well developed as compared with that region of Amiurus. Except for the 

 inclusion of the branches of the facialis between it and the alisphenoid cartilage 

 in Salmo, the anterior margins of the capsules are homologous, although the 

 processus postorbitalis is more pronounced in Salmo than in Amiurus. From 

 these comparisons it may be observed that Amiurus has an otic capsule which, 

 except for several minor differences, is typical of the teleostean condition. 



The parachordal region. Authors describing the origin of the chondro- 

 cranium in the teleosts have remarked that the basal plate of the older larva 



