VIII SKULL 27 
confluent with their respective epiotics. The floor and side walls 
of the inter-orbital section of the cranium are formed by a remark- 
able “sphenethmoid” bone which occupies the position of the 
paired ali- and orbito-sphenoids in other bony Fishes; and in one 
species, P. /apradei,' it forms in front distinct tubular investments 
round the olfactory nerves. In many respects this bone is singu- 
Fig. 132.—A, side view 
of the skull of Polyp- 
terus ; B, dorsal view, 
showing the chief 
dermal bones; C, 
similar view of the 
chondro-cranium after 
the removal of the 
dermal bones. An, 
Angular ; Av, articu- 
lar; D, dentary ; #£, 
mesethmoid ; Sm, 
foramen magnum ; 
Fy, frontal ; Z.e, lateral 
ethmoid ; Ma, max- 
illa; Na, Na’, nasal 
and accessory nasal 
bones ; occ, occipital ; 
ol, nasal aperture ; 
Op, operculum ; op.o, 
opisthotic ; O.f, os 
terminale ; Pa, pari- 
etal ; Pm.x, pre- 
maxilla; /P.t, post- 
temporal ; Pz, post- 
frontal; Qw, quad- 
rate; S.b, 8.6’, cir- 
cum-orbital ossicles ; 
S.Op, sub-opereulum ; 
Sp, splenial ; sp.eth, 
sphenethmoid ; spo, 
sphenotic ; Spr, 
spiracular ossicles, between which is the spiracle ; S.¢, supra-temporals ; J’, cheek-plate 
(pre-operculum); ¥’, ¥”’, smaller cheek-plates ; 2, 2, 2, 2, post-spiracular ossicles ; 
2, x, prespiracular ossicles. In C the cartilage is dotted. (From Traquair.) 
larly like the sphenethmoid bone of the Frog and other tailless 
Amphibia. A median ethmoid as well as lateral ethmoids are 
present. In addition to the ordinary dermal bones which invest 
the cranial roof there is a transverse row of supra-temporal 
plates crossing the cranial roof behind the paired parietals 
(Fig. 132, A). Fringing the outer margins of the frontals 
and parietals a row of pre- and post-spiracular ossicles extends 
nearly to the orbits, and between two of them, which form a 
1 Budgett, Trans. Zool. Soc. xv. 1900, p. 334. 
