52 ILUNOIS 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 intemasal 

 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 nasal sac. 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 intemasal space. The planum basale supports the 

 medial parts of the nasal sac and its ventral diverticulum. 



The planum tectale (/>/) 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 sup)erior prenasal processes extend downward and somewhat me- 

 dially into the intemasal 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 intemus 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 intemasal 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 (Jon). 



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 



