226 W. BALDWIN SPENCER. 



The walls of the foramina are lifted above the level of the 

 parietal bones, and it is perfectly possible, if not certain, that 

 the organ itself, enclosed in the eye capsule, projected con- 

 siderably beyond the surface. 



With the gradual extinction of these forms and of the 

 Deinosauria (i. e. Iguanodon, &c.), after the Cretaceous period 

 was passed, the organ, we may suppose, began with the rapidly 

 dwindling size of the specialised tertiary and later Reptilia and 

 Aves to lose its importance, until, degenerating in various de- 

 grees in different groups, it retained traces of its original eye-like 

 structure in the only groups in which, amongst living reptiles, 

 the parietal foramen persists ; its preservation being intimately 

 connected with and dependent upon the presence of this struc- 

 ture. The foramen is preserved amongst no group whatever 

 of existing Aves, and hence in these the epiphysis undergoes 

 considerable degeneration, though in its development it still 

 reaches a stage when, as in Reptilia, it consists of a distal 

 vesicle connected with the brain roof by a solid stalk. 



In Mammalia the degeneration is far more complete, and 

 all trace of the ancestral importance is completely lost. 



There now remains for consideration the two classes, Uro- 

 chorda and Cephalochorda; with regard to the latter it is 

 very difficult, if not impossible, to homologise any part of its 

 nervous system with the epiphysis of higher forms ; the per- 

 sistent anterior neuropore described by Hatscheck may perhaps 

 be homologous with that of other forms of Chordata which 

 closes during development, though even this must be regarded 

 as extremely doubtful owing to its position considerably 

 posterior to the anterior end of the notochord ; neither can it 

 for the same region be considered the homologue of the epi- 

 physis, which again lies posterior to the neuropore. The 

 azygos pigment spot described as an " eye " has no apparent 



the parietal foramen, presumably filled during life by the epiphysis. In addi- 

 tion to this, Professor Cope points out a large posterior process leading^back 

 towards the optic lobes and roof of the thalamencephalon, and which recent 

 work on living forms can scarcely leave room to doubt represents the flattened 

 pineal stalk. 



