20 ILLINOIS BIOLOGICAL MONOGRAPHS [20 



i 

 the olfactory organ; and together with the posterior prolongation of the 



cornu form a shelf upon which the posterior parts of Jacobson's organ rest. 

 Slightly anterior to its junction with the cornu, the tectale is pierced by a 

 small circular foramen, which, like the infra-conchalis of Amblystoma and 

 Triton, contains the anterior end of the organ of Jacobson. The planum 

 tectale is interrupted on its anterior half by two oval openings, of which the 

 external naris is the larger, extending back nearly to the middle of the cap- 

 sule. The more medial gap is much smaller and marks the beginning of the 

 process of resorption, just as in the older Triton larva. 



The lamina medialis (Im) is continuous with the tectale and forms the 

 medial wall of the capsule from the planum basale to the anterior cupola; 

 while ventrally, it curves into the floor of the capsule which is pierced by 

 two small foramina, through which rami of the nasalis internus nerve 

 leave the capsule. The main branch of this nerve passes to the internasal 

 space through the larger foramen just in front of the base of the pre-nasal 

 process. 



The adult Diemictylus (Fig. 15) shows no further chondrification of 

 structures than those present in the larva. Ossification has taken place in 

 all parts and further resorption has increased the size of the gaps present 

 in the earlier stage. A single foramen exists above the antorbital process, 

 so that both nasalis internus and externus of the profundus nerve enter the 

 capsule through the foramen orbito-nasalis. 



The planum tectale is more vaulted than before, and its posterior half 

 is pierced by two foramina; the medial of which is for a branch of the pro- 

 fundus nerve, while the lateral and larger one contains, as before, the 

 cephalic end of Jacobson's organ. The anterior tectale has two large 

 openings, the external naris, lateral in position, and, medial to this, the 

 further development of the gap begun by resorptive processes of the larva. 



The ventral surface of the adult capsule has a greatly enlarged choana, 

 which occupies the posterior two-thirds of the floor. It is bounded laterally 

 by the prolongation of the cornu, which in this stage extends beyond the 

 junction to the tectale, so that a short process, behind the shelf, supports 

 the posterior parts of the organ of Jacobson. The anterior part of the floor 

 and the cupola need no description, further than the mention of five small 

 foramina for branches of the nasalis internus nerve. 



The nasal capsules of Triton and Diemictylus are very close. The 

 presence of the same large gaps, and the reduced planum basale, together 

 with the continuity of cavum cranii with the internasal space are strong 

 resemblances between the two. The fenestra ethmoidalis is not a perma- 

 nent character of any other adult, although it is temporary in some larvae 

 as Salamandra and certain Anura. There are some resemblances between 

 the capsules of the adult Amblystoma and Diemictylus. In both, the 

 anterior part of the organ of Jacobson extends through a foramen which in 



