30 ILLINOIS BIOLOGICAL MONOGRAPHS [30 



The entire dorsal surface of the simple nasal sac and part of the lateral 

 surface is covered by a curved and fenestrated cartilage, more extensive 

 posteriorly where it curves ventrally over the choana. This process is a 

 further development from the columna ethmoidalis with large additions 

 from the cartilage cells described in the earlier stage, which now have 

 united to each other in such a way that small gaps expose the dorsal sur- 

 face of the nasal sac (Fig. 65). In front, the lateral parts of the fenestrated 

 process are lacking, so that the ethmoidal column alone forms the roof in 

 this region; while the more extensive chondrification occurs in the posterior 

 parts. This more complete development posteriorly, and the gradual 

 reduction toward the anterior end culminating in the unspecialized eth- 

 moidal column, indicates a development from behind forwards, and recalls 

 the condition in both Salamandra and Amblystoma in which the columna 

 ethmoidalis develops lateral processes, first from its posterior parts. 

 Although the fenestrated process of Necturus never unites to other capsular 

 parts, yet because of its relation to the ethmoidal column and the nasal 

 organ, it is the homologue of the planum tec tale of other Urodeles. 



The antorbital processes (pa) are larger than before, and although they 

 lie near the lateral parts of the fenestrated tectale, the two never unite, so 

 that a foramen orbito-nasalis is never completely enclosed; nor is there any 

 circumscribed olfactory foramen since the column is completely distinct 

 from the trabecula, and the olfactory nerve passes between column and 

 trabecula to the olfactory organ. 



The early development of the nasal capsule of Necturus resembles that 

 of other Urodeles to this extent; the trabeculae with their antorbital 

 processes, the planum basale and the ethmoidal columns are common to all. 

 On the other hand, Necturus lacks the characteristic expanded cornua, 

 the trabeculae ending bluntly, as well as a planum verticale and lamina 

 medialis, which in all other Urodeles completely separates the nasal organs 

 from each other and closes the internasal space from the cavity of the 

 forebrain. The nasal organs of Necturus are exposed on their ventral, 

 medial and lateral surfaces, and are only incompletely covered on the dor- 

 sal by the fenestrated cartilage. In the origin of the columna ethmoidalis 

 and its subsequent direction of growth, together with the development of 

 the fenestrated roof, there is a similarity to Amblystoma; but the many 

 striking contrasts between the capsules of these two Urodeles must estab- 

 lish very remote relationships between them, or are possibly to be inter- 

 preted by the neotenic character of this animal. 



The lack of skeletal protection for the nasal organs, the absence of 

 many parts of a typical urodelan capsule, as well as the very unspecialized 

 nasal sac itself, suggests for Necturus that it is to be regarded as either 

 ancestral, as a permanent larva or as indicating degeneracy of parts. To 

 regard Necturus with its fenestrated capsule, which has no counterpart 



