52 ILLINOIS BIOLOGICAL MONOGRAPHS [52 
the chondrocranium described by Gaupp (1893) of a 13 mm. Rana fusca. 
In this stage (Figs. 36, 37), a complete cavum cranii has been formed, 
bounded upon each side by the high alisphenoid cartilage, the internasal 
plate or planum basale forming the floor; while in front a perpendicular 
ethmoidal wall, pierced by a pair of olfactory foramina, separate the olfac- 
tory lobes from the nasalsac. Anterior to this ethmoidal wall, the planum 
basale (pb) forms the floor of the capsule; extending forward as a thick 
plate as far as the large naso-basales fenestrae, which open from the cavity 
of the capsule to the internasal space. The planum basale supports the 
medial parts of the nasal sac and its ventral diverticulum. 
The planum tectale (pt) forms the roof of the capsule; reaching forward 
from the cavum cranii, somewhat more narrow and thinner than the 
basale, it covers the medial portions of the nasal sac to the external naris, 
in front of which it bends abruptly downward to form with the solum 
anterius, the anterior wall of the capsule. At its anterior end, the tectale 
expands laterally into a pair of alinasal cartilages, which curve outward 
and then upward, supporting the anterior end of the nasal sac and forming 
the ventral margin of the external naris; while from the base of these a 
pair of superior prenasal processes extend downward and somewhat me- 
dially into the internasal space (Fig. 82). Just medial to the external 
naris, the tectale is pierced by a small foramen on either side for the exit of 
the internus branch of the fifth nerve. 
The planum verticale (pv) completely separates the nasal organs from 
each other. Arising from the median line of the perpendicular ethmoidal 
wall, it chondrifies anteriorly uniting the plana basale et tectale by their 
dorsal and ventral surfaces respectively. It extends forward to the tip 
of the skull, and its anterior margin separates the two large circular naso- 
basales fenestrae, which perforate the forward wall of the capsule (Fig. 80). 
These fenestrae surround the medial portion of the ventral nasal diverticu- 
lum together with the frontalis nerve, which passes to the intermaxillary 
gland in the internasal space. 
In contrast to Pipa, a posterior wall is formed in the capsule of Bufo, 
by a lateral extension of the posterior part of the planum tectale, which 
curves outward and downward beyond the alisphenoid, uniting to the 
anterior prolongation of the pterygoid, which has already fused with the 
lower parts of the cranial wall. In the angle formed by the fusion of the 
pterygoid, tectale and alisphenoid, a small orbito-nasalis foramen conducts 
rami of the fifth nerve into the capsule; so that here, as in Pipa, there is a 
resemblance to the Urodeles, which furnishes a clue to the homologies of 
these parts (fom). 
The united elements of lateral tectale and anterior pterygoid form a 
small rectangular cartilage which lies against the lateral surface of the 
nasal sac. This cartilage has been variously named the pars plana (Parker 
