108 
EDWARD PHELPS ALLIS. 
fishes described by Gegenbaur of such a groove existing 
independently of the nasal flap and its related nasal-flap 
furrow, and it is quite certain that the groove is simply a 
secondary differentiation of the furrow. Such being the case 
the nasal-flap furrows of all the Plagiostomi are strictly 
homologous structures, and this is the conclusion that 
Luther (1909) arrives at from physiological considerations. 
A continuous nasal velum would then be formed if the 
furrows of opposite sides were to coalesce in the median 
line by the complete or partial breaking through of the 
intervening frenulum. There is, however, no indication 
whatever, in any of my specimens, that this frenulum is 
ever broken through, for even in Myliobatis the oral edge 
of the frenulum forms a part of the upper lip of the fish 
and not a part of the nasal flap of either side. The 
nasal-flap furrows of opposite sides are here certainly in 
communication with each other beneath the velum, but it 
is through the intermediation of the small persisting median 
section of the upper labial sulcus and not because of the 
coalescence of the furrows. I am accordingly convinced that 
a complete velum, extending across the median line, must, if 
ever found, be formed by the coalescence of the opposing- 
mesial edges of the nasal flaps of opposite sides in fishes 
where those flaps have been prolonged beyond the oral edge 
of the upper lip ; and this would seem to be confirmed by 
the conditions that I find in a small specimen of Scyllium. 
In this small specimen of Scyllium, which I am quite 
certain is Scyllium canicula, I find the nasal flaps of 
opposite sides so much more developed than those shown in 
Gegenbaur’s figure of this fish that it would seem as if the 
two fishes could not be of the same species. The flaps of 
opposite sides are separated by a small median incisure which 
extends to the oral edge of the frenulum, that edge certainly 
representing a small persisting median portion of the upper 
lip. There is accordingly no complete velum in my specimen 
of this fish. Such a velum would, however, be formed if the 
adjoining edges of the incisure were to fuse, and this is 
