54 ILLINOIS BIOLOGICAL MONOGRAPHS [Si 



HYLA PICKERINGII 



My work on Hyla has been restricted to a study of the adult, in which 

 the development of membrane bones over the anterior end of the forebrain 

 has been accompanied by a partial resorption of the posterior parts of the 

 capsule, so that homologies are not always certain. Still Hyla in general 

 has a somewhat close resemblance to its arciferan relative, Bufo. 



Behind the planum verticale {pv), the planum basale {ph) has under- 

 gone resorbtion in the middle line, so that it is represented by a pair of 

 narrow plates (Fig. 38). In front, however, the continuity of the two 

 sides persists to the anterior end of the capsule, where the planum basale 

 passes into the solum anterius. The perpendicular ethmoidal wall is 

 lacking, as well as the posterior part of the planum tectale, so that the 

 latter, posteriorly is reduced to an oblique bar on either side, the tectale 

 being complete only in its anterior third, where it is expanded in front to 

 twice its width. At the tip of the cranium, the tectale bends downward to 

 pass into the vertical solum anterius. 



All that remains of the planum verticale {pv) is a median septum, with- 

 out a gap, uniting the anterior parts of plana basale et tectale. In front, 

 where it joins the solum anterius, the tectale separates the two basal fenes- 

 trae, which are very much smaller than in Bufo. Dorsal and slightly 

 lateral to these openings, is a small foramen for the exit of the nasalis 

 internus nerve from the capsule. 



The lamina externa {le), developed partly from the lateral part of the 

 planum tectale, and partly from the anterior prolongation of the pterygoid 

 process, forms the side wall of the capsule in the choanal region. Anterior 

 to its connection with the tectale, this plate narrows considerably, and is 

 interrupted by an oval gap, apparently a further evidence of resorbtion, as 

 no sensory or nervous structures were found to pass through it. Just 

 anterior to this opening the oblique cartilage {oc) passes upward and for- 

 ward from the lamina externa over the nasal sac, to imite to the planum 

 tectale at its widest part, its ventral side being expanded to form the pos- 

 terior boundary of the naris (Fig. 77). 



Just ventral to the origin of the oblique cartilage from the lamina 

 externa, the lower margin of the latter expands outward into a small 

 process which separates the cavum inferius from the more ventral cavum 

 medium. More anteriorly, this part of the lamina externa unites with the 

 lateral parts of the planum basale to form the solum anterius, which 

 constitutes the anterior wall of both cavum inferius and c. medium, and the 

 more medial parts of the capsule. 



As in Bufo, the solum anterius is a thin plate of cartilage which curves 

 upward and continues into the anterior margin of the planum tectale. 

 Near its junction to the latter, a small alinasal cartilage extends laterally, 

 forming the anterior boundary of the external naris; it is continuous med- 



