374 
EDWARD PHELPS ALLIS. 
where the constrictor never separates into superficial and 
deeper layers, the nerve apparently always lies on the external 
(anterior) surface of the muscle. 
In the several figures given by Ruge, the nervus facialis 
is seen to lie internal to the dorsal portion of the hyal con- 
strictor superficialis only in Heptanchus and possibly, in part, 
in Spinax ; the nerve in the latter fish first lying on the 
external surface of the muscle and then apparently piercing 
it before it, the nerve, reaches the level of the hyomandibulo- 
ceratohyal articulation. In all of the many excellent figures 
of these fishes given by Luther (1909), the nerve lies internal 
to this part of the constrictor only in Heptanchus, Hexanchus, 
and Lamna. In the remnant of a head of Lamna that I have, 
I find the anterior fibres of the proximal portion of this part 
of the constrictor inserted on the palatoquadrate, but the 
remaining proximal fibres inserted on the liyomandibula. 
The nervus facialis lies internal to those fibres that are 
inserted on the palatoquadrate, but, beyond those fibres, it 
lies between the palatoquadrate and the hyomandibula, and 
hence external to the fibres inserted on the latter cartilage. 
In all the other Selachii figured by both Ruge and Luther, 
the nervus facialis lies external to all the fibres of this portion 
of the hyal constrictor. 
In Heptanchus and Hexanchus the liyomandibula is 
relatively slender and lies internal to the palatoquadrate 
(Gfegenbaur, 1872). In Lamna I find the dorsal end of the hyo- 
mandibula lying internal to the palatoquadrate. In all the 
other Selachii figured by Ruge and Luther, the dorsal end of 
the hyomandibula, so far as I can determine from existing de- 
scriptions at my disposal, lies posterior to the palatoquadrate 
and separated from it by a considerable interval, as shown in 
Gregenbaur’s figures of Mustelus, Scymnus, Centrophorus, 
and Heterodontus. This, then, probably gives an explanation 
of the differing relations of the nervus facialis to the dorsal 
portion of the hyal constrictor. Where the dorsal end of the 
hyomandibula lies internal and close to the palatoquadrate, 
the nervus facialis also lies internal and close to the 
