57] THE NA SA L ORGA N IN A MPHIBJA HIGGINS 57 



causes a shortening of the capsule, which is accompanied by an increase in 

 its depth. In front, each trabecula gives rise to a solum anterius which 

 forms the anterior wall of the capsule; and at its junction to the tectale 

 each solum gives rise to an alinasal cartilage, which supports the anterior 

 end of the nasal sac and forms the posterior boundary to the external naris. 

 The large naso-basal fenestrae between the medial margins of the anterior 

 walls of the capsule and the planum verticale open from the nasal cavity 

 to the internasal space much as in Bufo. 



In this stage, the articulation of the lower jaw has moved backward, so 

 that the deeply curved muscularis process of the quadrate is more poster- 

 ior; and, as a result, the pterygoid has united to the capsule at the point of 

 origin of the tectale from the ethmoidal column, the anterior maxillary 

 process reaching forward from the junction of the two. 



In a young adult of Rana mridescens (Figs. 85, 86) there has been a 

 relative reduction in the height of the verticale, although the proportion 

 of breadth to length is approximately unchanged. Anterior to the cavum 

 cranii, planum tectale, basale and verticale have fused into a thick plate, 

 recalling the association of these same structures in Amblystoma. The 

 olfactory foramen (Jo) opens obliquely forward from the antero-lateral 

 angle of the cavum cranii to the nasal cavity just beneath the foramen 

 orbito-nasalis the posterior opening of which lies adjacent to the lateral 

 margin of the cranial wall. Lateral to the foramen orbito-nasalis, the 

 fused tectale and antorbital curve outward and downward over the choana 

 and continue posteriorly into the pterygoid, while anteriorly a small maxil- 

 lary process reaches forward midway to the tip of the capsule. . 



The nasal sac is completely covered dorsally by the lateral parts of the 

 planum tectale which reaches forward to the tip of the cranium, and is 

 considerably narrower at the anterior end. At about the middle of the 

 tectale, an oblique cartilage bar (oc) extends backward and slightly out- 

 ward, ending bluntly near the anterior end of the maxillary process (amp). 

 Although unconnected in this stage, these two structures are probably 

 the homologue of the lamina externa and the oblique cartilage of the larva. 



The nasal sac opens to the exterior just anterior to the oblique cartilage, 

 where the alinasal cartilage, continuous anteriorly into a superior prenasal, 

 forms the floor of the naris. Beneath the alinasal, but connected to it, is 

 a thick cartilage, the crista subnasalis (cr s) possibly a remnant of the 

 earlier solum anterius, although evidence is lacking here. Anterior to the 

 crista sub-nasalis is the inferior prenasal process, which extends ventrally 

 from the capsule; while directly above it is the fenestra naso-basalis, 

 greatly reduced here, and more like that in Hyla. 



