SKULL 



227 



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. lapradei} it forms in front distinct tubular investments 

 round the olfactory nerves. In many respects this bone is singu- 



Fi«. 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 ; Ar, articu- 

 lar ; Z>, dentary ; K, 

 mesethmoid ; /.m, 

 foramen magnum ; 

 i^r, frontal ; Z.e, lateral 

 ethmoid ; Mx^ max- 

 illa ; Na^ Na\ nasal 

 and accessory nasal 

 bones ; oec, occipital ; 

 ol, nasal aperture ; 

 0^, operculum ; op.o, 

 opisthotic ; O.t^ os 

 terminale ; Fa, pari- 

 etal ; Pw.x, pre- 

 niaxilla ; P.t, post- 

 temporal ; Ptf, post- 

 frontal ; Qu, quad- 

 rate ; S.b, S.i', cir- 

 cum-orbital ossicles ; 

 S.Op, sub-operculum ; 

 ,Sp, splenial ; sp. eth, 

 sphenethmoid ; sp.o, 

 sphenotic ; ''^pr, 



spiracular ossicles, between which is the spiracle ; ,S.t, supra-temporals ; Y, cheek-plate 

 (pre-operculum) ; I", i"', smaller cheek-plates ; z, z, z, z, post - spiracular ossicles ; 

 z', z', 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. 1.32, 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. Hoc. xv. 1900, p. 334. 



