27] THE NASAL ORGAN IN AMPHIBIA HIGGINS 27 



PLETHODON ERYTHRONOTUS 



Winslow (1898) has described the nasal capsule of a 20 mm. larva of 

 Plethodon glutinosus, and he compares it with a 45 mm. Amblystoma. I 

 have not studied any larval form of this genus, but have examined and 

 modelled the capsule of an adult, which is similar in many respects to my 

 last stage of Amblystoma. In the adult, ossification is extensive and in 

 some parts, all traces of a chondrocranium have disappeared. This is 

 especially true in the region of the eye, where the trabecular crests have 

 disappeared as far forward as the olfactory foramina, so that these open- 

 ings are confluent with the foramina orbito-nasales in this stage (Fig. 27). 



The dorsal surface of the nasal organ is exposed throughout its entire 

 length by a large gap in the capsule, similar to that of the adult Amblys- 

 toma; so that the planum tectale is reduced to a small cartilage plate 

 covering the posterior end of the nasal sac behind the choana. The lamina 

 externa (le) formed by elements of the tectale and processus antorbitalis, 

 passes obliquely forward as a band of cartilage, protecting the lateral parts 

 of the nasal sac and the organ of Jacobson (Fig. 62). The nasalis externus 

 of the profundus nerve enters the capsule through a small foramen in this 

 plate, and is partly distributed to Jacobson's organ; while the main branch 

 of the nerve leaves the capsule through the foramen in the more anterior 

 part of the capsule. Just lateral to the foramen nasalis externus (fne), 

 the lamina externa abruptly expands from its ventral margin into a horizon- 

 tal plate which partly covers Jacobson's organ, its lateral angle terminating 

 in a small anterior process; while more anteriorly it narrows considerably 

 and lies directly above the posterior prolongation of the cornu (Fig. 63). 

 The anterior medial angle of the lamina externa is united by a cylindrical 

 bar to the anterior cupola, as in the adult Amblystoma (Fig. 10), which 

 separates the external naris from the large gap in the dorsal surface. The 

 naso-lacrimal duct passes to the nasal organ through the notch between 

 this bar and the anterior part of the externa, which is pierced by the fora- 

 men for the exit of the nasalis externus, as mentioned above. 



The floor of the capsule is formed by the cornu trabeculae, which has 

 united to the lateral margin of the lamina externa, and continues poster- 

 iorly as a short process supporting the anterior end of the organ of Jacob- 

 son. The ventral gap in the capsule is very large and is similar to that in 

 Amblystoma. 



The planum basale and verticale are united throughout their entire 

 length, forming a thick plate which unites the capsules just anterior to the 

 olfactory foramina and separates the internasal space from the cavum 

 cranii. Anterior to these parts, the lamina medialis (Im) forms the medial 

 wall of the capsule; narrower at its junction to the verticale, it becomes 

 wider more anteriorly and finally uniting with the cornu trabeculae, it forms 

 the anterior vault of the capsule. Midway between the tip of the capsule 



