28 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, 



physiological significance. In Ceratodus there is the further phy- 

 siological relation to aerial respiration ; and in all the higher Verte- 

 brata the nasal passages are concerned in sniffing and breathing. 



"With respect to the internal structure of Ceratodus^ I shall con- 

 fine my remarks, in the present communication, to the brain, the 

 skull, and the pectoral limbs. Ceratodus is, in fact, the most sur- 

 prisingly suggestive animal I have ever had occasion to study ; and 

 the attempt to comprehend the morphological significance of the 

 organs I have mentioned has led me so far, that I must defer the 

 consideration of other parts of its organization to another occasion. 



I. The Brain. 

 I had no great hope of finding the brain in a state fit for ex- 

 amination in my specimen of Ceratodus ; and in fact the cerebral 

 substance and that of the nerves are in a very friable condition. But, 

 by great good fortune, the pia mater is so very dense and tough, 

 that it has held the cerebral substance in place ; and thus not only 

 the external form, but somewhat of the internal structure of the brain 

 could be satisfactorily determined. 



Fig. 3. 



-J.K X 



Ceratodus forsteri. Underview of the brain (nat. size). 

 I, olfactory, II, optic, VI, third nerve (?) ; V, VII, IX, X, roots of the tri- 

 geminal, portio dura, auditory, glossopharyngeal, and pneumogastric nerves ; 

 v.c, one of the lateral ventricles of the lobus communis, laid open from below. 



The brain is represented from above in fig. 1, from the left side 

 in fig. 2, and from below in fig. 3. 



