270 IOWA GEOLOGICAL SURVEY 



that it was primarily derived from a modified integumentary 

 sense organ, such as the lateral line system; that it gradually be- 

 came more deeply seated within the skull, at the same time divid- 

 ing into two parts, utriculus and sacculus, the former of which 

 gave rise to the semicircular canals, and the latter to the tube- 

 like ductus endolymphaticus and the lagena (=cochlea). In 

 some cases, however, as in certain sharks and rays, the separa- 

 tion between the utricular and saccular chambers remains in- 

 complete, and their common cavity is known as the utriculo- 

 saccular organ. Other primitive features retained among Elas- 

 mobranchs are the simpler relations of the semicircular canals 

 to the utriculo-sacculus, and the opening of the endolymphatic 

 duct (closed in higher fishes) upon the dorsal surface of the 

 head, where it connects with the lateral line canals.* 



In view of the great antiquity and lowly systematic position 

 of the Palaeoniscid species we are considering, one might expect 

 that the auditory sense organs would betray signs of a primi- 

 tive condition approximating to that observed among sharks 

 and rays, or would at least manifest simpler relations than those 

 we are familiar with among modern bony fishes. In this expec- 

 tation, however, we are disappointed. The type of membranous 

 labyrinth which is preserved for us with utmost nicety of detail 

 in these ancient Palaeoniscids is to all intents and purposes 

 identical with that in existing teleosts. No really essential dif- 

 ferences can be detected, and we are obliged to affirm that these 

 early ganoids were provided with as efficient auditory sense 

 organs as any fishes now living. The faculty of equilibration 

 must have been perfectly developed among them, and for aught 

 that can be inferred to the contrary, they may even have pos- 

 sessed the power of hearing, a sense that some authors regard as 

 the most delicate and most recently acquired in point of evolu- 

 tionary sequence among the various means of receiving sense 

 impressions from the external world. 



From these general observations we may pass on to a more 

 particular description of the internal ear, as displayed by a 



* References to the principal literature of this subject are brought together by 

 J. B. Johnston, in his manual on "The Nervous System of Vertebrates" (1906), by 

 G. Schwalbe, in his "Lehrbuch der Anatomie der Sinnesorgane" (1887), and with 

 still greater fulness by the authors of special monographs on the vertebrate ear, 

 foremost among whom is the Swedish anatomist, Retzius. 



