22 ILLINOIS BIOLOGICAL MONOGRAPHS [22 



far described, in the fact that the dorsal surface is not concave but is convex 

 and bears upon its anterior half a distinct median swelling which probably 

 is the beginning of a planum verticale (Fig. 58). The anterior surface is 

 straight and at right angles to the median surface of the skull, while the 

 posterior possesses a short caudal process similar to that in the 25 mm. 

 Salamandra larva. 



Anterior to the planum basale, each trabecula extends forward a short 

 distance and from its medial margin a columna ethmoidalis (ce) arises 

 dorsally as a broad band of cartilage, partially separating the anterior 

 part of the nasal sac from the internasal space. Passing backwards, up- 

 wards and outwards along the dorsal medial side of the olfactory organ, 

 each column becomes more rod-like; and at the level of the anterior margin 

 of the planum basale it expands into a large rhomboidal plate, the planum 

 tectale (pi) which extends forwards and outwards, covering the dorsal and 

 lateral parts of the nasal sac. Each tectale is pierced by a small foramen 

 through which a branch of the profundus nerve passes from the capsule. 

 Posterior to the tectale, each column continues backward and unites to the 

 crista trabeculae. 



The cornu trabeculae (ct) of Cryptobranchus differs from that of any 

 other Urodele. In contrast to the board triangular plate of both Ambly- 

 stoma and Salamandra, it is reduced to a narrow bar, which curves back- 

 wards and upwards from the lateral anterior margin of the trabecula to a 

 point just below the tectale, to which it fuses in a later stage. A small 

 notch at the anterior end of the capsule, between the ethmoidal column 

 and the cornu allows for the passage of the nasalis internus nerve to the 

 internasal space. 



From each trabecula, just posterior to the large foramen olfactorius, a 

 slender process passes laterally a distance equal to the width of the trabe- 

 cula, and then bends abruptly to pass backward to the pterygo-quadrate, 

 while it utterly lacks any anterior prolongation. The morphological 

 relations of this bar will be discussed in connection with a later stage. 



In a larva two months after hatching, a more complete capsule has 

 formed. The planum basale has lost the hinder process and is now con- 

 tinuous with the planum verticale, which unites the two capsules just 

 anterior to the oval olfactory foramina and completely separates the 

 cavum cranii from the internasal space. Anterior to the verticale, the 

 columna ethmoidalis and cornu trabeculae have expanded to form the 

 lamina medialis covering the entire medial surface of the nasal organ. A 

 notch, marking the junction of column and cornu, allows for the passage of 

 a branch of the nasalis internus to the internasal space. 



The planum tectale is considerably larger than before, reaching back to 

 the crista and extending forward over the posterior tip of the cornu trabe- 

 culae, with which it is not yet united. It is pierced by a small foramen as 



