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 Sglamandra 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 tectale 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 
