LOWER OLD RED SANDSTONE OF SCOTLAND. 337 



separated from it by but a thin film. Now, it is a curious fact, 

 tliat in an ancient ganoid of the Lower (Middle) Old Red 

 [Spec. 4], still unfurnished with a name, we may detect organs 

 of hearing akin to those of the placoids, — more especially to 

 those of the sharks. Under this occipital plate we find the 

 upper part of a brain-chamber, surrounded by strong walls of 

 bone ; immediately behind, we find the chambers of the in- 

 ternal ears, with fragments of the comparatively slim parti- 

 tions which separated them from that anterior portion of the 

 spine which traversed the posterior part of the occiput. Ex- 

 actly as in the shark, too, we have the auricular passages open- 

 ing backwards, and presenting at their outer end the same 

 angular form as in the dog-fish. It is surely interesting to 

 be thus enabled to determine that the earliest ganoids of 

 which we know anything were connected by very striking 

 affinities to the placoids. There are several of the speci- 

 mens on the table that exhibit the openings through which 

 the eyes once looked out [Spec. 5]; and there are a good 

 many more in my collection at home. In Pterichthys even 

 the capsules are occasionally preserved. They are preserved 

 in a Gamrie specimen figured in the "Geological Journal" 

 for November 1848, by Sir Philip Egerton, and in a Dura 

 Ben specimen belonging to Mrs Bonar of Cupar Fife [Spec. 6], 

 of which she has kindly given me the use. The form of the 

 eye-orbit in these ganoids was various : in the Pterichthys it 

 was circular ; it approximated in the Asterolepis to triangu- 

 lar ; it was lentiform in the Coccosteus, and elliptical in the 

 Diplopterus. I need scarce remind the Association that the 

 elliptical and lenticular forms are the prevailing ones among 

 existing placoids. In short, to conclude this part of my sub- 

 ject, we have in the specimens before us evidence of at least 

 three senses with which these ancient fishes took note, in an 

 incalculably remote period, of the sights, sounds, and odours 

 of the material world. 



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