DEVONIAN FISHES OF IOWA 269 



Most anteriorly of all is the region known as the prosencephalon 

 or forebrain, represented in Fig. 40 A by a slender elongated 

 process which may perhaps be regarded as identical with the 

 olfactory lobes, although its details are not entirely clear. 



From the description of parts just given, as well as from the 

 appearance of the surface of the organ when examined under 

 the lens, it is evident that we have here to deal with a veritable 

 brain-structure the substance of which became transformed into 

 calcium phosphate before decomposition could set in, and whose 

 walls in consequence are scarcely shrunken. This view is fur- 

 ther confirmed by the presence of nerve fibres and blood vessels, 

 slightly enlarged in some cases, it is true, by the segregation of 

 mineral matter, but coinciding in position with altogether simi- 

 lar nervous and vascular structures in modern ganoids and 

 bony fishes. The case, therefore, is entirely different from the 

 state of affairs which arises from infiltration of the brain cavity, 

 thus producing a natural cast of the interior, and so far as the 

 present writer is aware the phenomenon is unparalleled in the 

 annals of palaeontology. We may be permitted to refer in pass- 

 ing to an interesting dissertation on fossil brain casts, to be 

 found in chapter 7 of Professor Albert Gaudry's "Essai de pale- 

 ontologie philosophique" (Paris, 1896), wherein are considered 

 examples of all the higher classes of vertebrates. 



We pass now to an examination of the most interesting of all 

 the internal structures revealed by the heads of those specimens 

 which have been fractured in such manner as to disclose the con- 

 tents of that long-sealed abode, which, eloquent of 'silence and 

 slow time,' recalls Keats' apostrophe of a Grecian urn: 



Thou still unravish'd bride of quietness, 



Thou foster-child of silence and slow time, 



Thou, silent form, dost tease us out of thought 

 As doth eternity: cold Pastoral! 



The structures referred to are the internal ear and its asso- 

 ciated parts; and, as in the case of the brain itself, their like 

 has never before been found in the fossil condition. As will 

 presently appear, they are nearly as well suited for purpose of 

 investigation as alcoholic preparations, and naturally are of 

 surpassing interest on account of their antiquity. Concerning 

 the origin of the auditory sense organ, embryology teaches 



