PAPERS ON ZOOLOGY 297 



pierces the posterior ^vall of tlie nasal capsule and con- 

 ducts tlie rami of the fifth nerve into the nasal cavity. 

 In the chondrocranium of Bufo americana, these same 

 nerves pierce the posterior wall of the capsule just above 

 the pterygoquadrate, where it joins the cranium; the 

 same is true in Eana and Pipa, although in the latter, 

 some modifications in the capsule have occurred, making 

 the comparisons to the Phaneroglossa more remote. 

 Since it is customary to regard nerves and their distri- 

 bution as sufficient criteria for the determination of 

 homologous structures, it would apparently follow that 

 the orbito-nasalis foramen has been reduced from the 

 large gap between the planum tectale and the antorbital 

 process ; and that these foramina in the Urodeles and the 

 Anura occur in homologous parts. Accordingly that 

 portion of the posterior wall of the nasal capsule dorsal 

 to these nerves must be a derivative from the planum 

 tectale, while the cartilage immediately beneath them is 

 as clearly antorbital in origin, or in the Anura and Cryp- 

 tobranchus clearly pterygoidal. 



SUMMABY 



The antorbital process of the Urodela arises from the 

 lower margin of the trabecula, considerably back of the 

 choana ; during the later stages it unites with the struc- 

 tures farther forward, and forms a part of the posterior 

 wall of the nasal capsule. In Cryptobranchus alleghan- 

 iensis the pterygoquadrate arch joins the side of the 

 trabecula in the position from which the antorbital nor- 

 mally develops in other Urodela; and in this respect 

 CryiDtobranchus more closely approaches the Anura, in 

 which the junction of the pterygoquadrate to the cranium 

 is the characteristic condition. 



The rami of the fifth nerve pass into the nasal cap- 

 sules of the Urodela through an orbito-nasal foramen, 

 dorsal to the antorbital process; while in the Anura, 

 these same nerves pass through a foramen just above 

 the pterygoid process, where it joins the cranium. That 

 part of the posterior wall of the nasal capsule in the 

 Urodela, ventral to these nerves is clearly antorbital; 



