Brain and Pineal Structures of Polyodon folium. 307 



mid-brains justify this impression, as do the condition 

 of the pineal structures and their relation to the exterior. 

 The brain of A. ruhicundAis is certainly that of a more 

 active and intelligent fish. The corresponding nerves are 

 larger in the sturgeon ; its fore-brain is larger relatively ; 

 the mid-brain is larger and with an evident suggestion 

 of doubleness; the medulla is smaller relatively; and, 

 finally, the pineal end organ is less evidently placed and 

 surrounded to receive impressions from without. 



EXPLANATION OF FIGURES. 



Plate I. ^^ 



Fig. 1. Side view of brain and pineal structures of 

 Polyodon as they appear when the cartilage is cut away 

 from one side. Natural size. Magnus Westergren del. 



Plate II. 



Fig. 2. Dorsal view of brain of Polyodon, with the 

 connective tissue and fat removed from the fore- and 

 mid-brains and the cerebellum, but with membrane roof 

 of 4th ventricle left in place. A, nasal sac; B, olfactory 

 lobe; C, pineal stalk; D, fore-brain; I, mid-brain; J, cere- 

 bellum; M, medulla oblongata; O, spinal cord; 1, olfac- 

 tory nerve; 3, oculo-motor nerve; 10, vagus nerve. 



From a photograph. 



- , \ 



Plate III. 



Fig. 3. Dorsal view of brain of Polyodon. B, olfac- 

 tory lobe; C, pineal stalk; D, fore-brain; I, mid-brain; 

 J, cerebellum ; K, valvula cerebelli ; L, restiform tract ; 

 0, spinal cord ; P, part of membrane roof of medulla 

 oblongata, turned aside; 1, olfactory nerve; 2, optic 

 nerve; 3, oculo-motor nerve; 4, trochlear nerve; 5, 5, tri- 

 geminal nerves; 7, facial nerve; 8, auditory nerve; 9, 

 glosso-pharyngeal nerve; 10, vagus nerve. H. Garman 

 del. 



