1c0 EDWARD PHELPS ALLIS, JR. 
Plagiostomi, that it hes on what is, in them, a part of the ex- 
ternal surface of the snout, which has here been secondarily in- 
cluded in the buccal cavity. There is no naso-buceal groove 
connecting this aperture with the upper edge of the primary 
cavity of the mouth, The fold of the secondary upper lip, 
passing as it does between the two nasal apertures, might be con- 
sidered to correspond to the lateral edge of the snout of Chlamy- 
doselachus, which also passes between the two apertures, but 
these two edges are not homologous, for they both exist con- 
temporaneously and independently in Chlamydoselachus. 
In Ginglymostoma concolor and Stegostoma fasciatum the 
fold of the secondary upper lip has, as shown in Miller and © 
Henle’s (41) figures, approximately the course which it has in 
Heterodontus, but it apparently crosses the nasal pit slightly 
oral to the process 8, and the fronto-nasal flap is not so well 
defined as in Heterodontus. 
It is commonly said of Heterodontus, that the nasal and 
buceal cavities are confluent, that the two nasal apertures are 
connected by a naso-buccal groove, or that the excurrent aper- 
ture has shifted orally until it has cut through the upper lip 
and so come to lie on the internal surface of the lip; the upper 
lip and the buccal cavity of this fish being considered to be the 
strict homologues of the lip and cavity of other Plagiostomi. 
These assumptions are, however, all incorrect. 
Huxley (76) considered the excurrent aperture of this fish as 
formed by the incomplete bridging of a naso-buccal groove, and he 
compared it with the posterior nasal aperture of Ceratodus. This 
implies two assumptions; first, that this aperture of Heterodon- 
tus represents the oral end of a naso-buccal groove which has been 
incompletely bridged by the arching over of its opposite edges; 
and, second, that it.is the homologue of the posterior aperture 
of Ceratodus, the latter aperture then being the oral end of a 
canal formed by the completed bridging of a naso-buccal groove 
similar to the one assumed to be found in Heterodontus. The 
first of these two assumptions is incorrect, as explained above. 
The second assumption is probably correct in so far as the 
homology of the posterior nasal apertures of Heterodontus and 
