LIPS AND NASAL APERTURES IN FISHES 189 
and finally reaches and opens into the depression in the roof of 
the mouth, there forming the primitive choana. 
The lateral process of these embryos of Hypogeophis thus 
fuses in two places with the fronto-nasal process, one internal 
to the canal leading from the nasal pit to the primitive choana 
and the other external to it, the canal thus lying between the 
two points of fusion and leading primarily from the definitive 
external nasal opening to the choana without traversing the 
nasal pit. This canalthus strongly recalls the passage which, in 
the adult Chimaera, lies between the valvular nasal process and 
the nasal portion of the naso-labial fold and leads from the 
antero-mesial nasal aperture to the postero-lateral one, and I 
consider it to be its homologue. 
If this be so, the conditions in Hypogeophis would be derived 
directly from those in Chimaera by the fusion, first, of the 
valvular process of Chimaera with its valve-seat, this forming a 
nasal bridge and being represented in Hypogeophis by the first 
of the two fusions of the lateral and fronto-nasal processes, and 
second, by the fusion of the nasal portion of the naso-labial fold 
of Chimaera with the premaxillary lip, this being represented in 
Hypogeophis by the second of the two fusions of the two proc- 
esses. By an extension of the first of these two fusions, the 
postero-lateral nasal aperture of Chimaera is occluded, this 
leaving the naso-buccal groove external to the nasal bridge and 
an oral remnant of it persisting as the depression in the roof of 
the mouth of Hypogeophis. The large lateral process of embryos 
of Hypogeophis thus represents, in its deeper portion, the lateral 
nasal process of the Plagiostomi, and in its superficial portion 
the nasal portion, of the nasolabial ‘fold of Chimaera. The 
primitive choana of Hypogeophis would then correspond to the 
oral end of the naso-buccal groove of Chimaera, and would 
accordingly be the homologue of the choana of the Amniota, 
but its connection with the nasal pit would be by a canal which 
passes external to the nasal bridge instead of internal to it. 
In the Urodela and Anura the conditions differ from those in 
the Gymnophiona simply in that the shallow groove which 
becomes the choanal canal of the latter has been obliterated 
