194 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
moistening the olfactory epithelium. Usually there is little complica- 
tion of the olfactory surface, but in a few urodeles (Plethodon) there 
is a projection from the lateral wall, the first indication of the conche 
which acquire such development in the higher groups. There is fre- 
quently a differentiation of the nasal passage into a ventral respiratory 
duct lined with ordinary and a more dorsal olfactory duct lined with 
sensory epithelium. In the lower urodeles the diverticulum repre- 
senting the organ of Jacobson is on the medial side of the nasal cavity; 
a little higher it is ventral, while in the highest urodeles it has rotated 
to the lateral side. It may be noted that some of the amphibia have 
smooth muscles to close the external nares. 
Aside from the varying position of the choane the changes from 
amphibia to reptiles in the olfactory organs are comparatively slight. 
Fic. 198.—Longitudinal section of nasal region of alligator, after Gegenbaur. c, concha; 
ms, maxillary sinus; 1, naris; p, pseudoconcha. 
The olfactory region becomes more distinct from the respiratory tract 
and the latter shows a tendency to be differentiated into an anterior. 
atrium or vestibule, a middle area connected with the olfactory region, 
and a posterior naso-pharyngeal duct between the basis cranii and 
the roof of the mouth. This latter duct varies in length accordingly 
as the choane are anterior or posterior in position, the extreme being 
reached in the crocodiles, where by ingrowth of palatines and ptery- 
goids, the internal nares are carried back nearly to the hinder end of 
the skull. A single concha, supported by bone, is developed in the 
lateral wall of the reptilian nose. It is weak in the turtles (fig. 197), 
but is larger elsewhere, and in the crocodiles (fig. 198) it becomes 
divided in front, while a ‘pseudoconch’ (its homology with the supe- 
rior concha of birds is uncertain) is developed above and behind the 
true concha. Jacobson’s organ occurs only in the squamata, where 
it forms a simple pocket in the primitive position, ventral and medial 
to the nasal cavity, near the nasal septum. 
