VEETEBEATES FROM A ORUSTAOEAN-LIKE ANCESTOR. 383 



Durham and Mr. Griinbaum for making me photographs of 

 various specimens, which have been of great assistance in 

 making the drawings ; to Mr. Shipley for making the series 

 of drawings in PI. XXVI, figs. 13a— 13e; to Mr. Hardy and 

 Mr. Shore for assistance in the preparation and staining of 

 material ; and to Dr. Hunter for valuable advice in connection 

 with the formation of pigment. To all these gentlemen I beg 

 here to oflFer my sincere thanks. 



The description of the brain of the adult Petromyzon as 

 given by Ahlborn (3) is so thorough and so excellent, that it 

 is hardly necessary for me to re-describe the arrangement of 

 its different parts, especially as the brain of the Ammocoetes 

 resembles very closely that of the adult form ; the main dif- 

 ference being, as Ahlborn has pointed out, that the internal 

 cavities are more roomy, and that the optic lobes are not so 

 fully developed. I shall, therefore, refer my reader to his 

 descriptions and illustrations, and only give such illustrations 

 from my own specimens as seem to me necessary to make clear 

 the arguments brought forward in this paper. 



In PI. xiii of Ahlborn^s paper (3) drawings are given of 

 the brain from different points of view : as, however, he lias 

 removed the choroid plexuses in all his drawings I have 

 thought it best to give a drawing of the dorsal aspect of the 

 brain as it appears in the fresh condition when the brain-case 

 is removed. The specimen from which fig. 1, PI. XXV, was 

 taken measured 120 mm. in length, and the brain was care- 

 fully dissected out under a dissecting lens, and then the draw- 

 ing was made. The greater portion of the dorsal surface is 

 taken up with the transparent folds of the choroid plexuses 

 ii and iii ; the folds of these two plexuses meet, as 

 described by Ahlborn, over a deep fissure, which indicates the 

 position of the band of fibres constituting the cerebellum. 

 In front of the folds of the choroid plexus ii are seen, in the 

 middle line, the large r. gangl. habenulse, and the white opaque 

 pineal eye with the masses of the cerebral hemispheres on 

 each side. On the ventral side (see Ahlborn) the brain is 

 divided by a marked constriction into a pre-chordal and an 



