284 COMPARATIVE ZOOLOGY OF CHORDATES 



supports. The dermal or membrane-bones form a covering 

 just beneath the skin, and in certain regions they line the 

 mouth-cavity. The external covering of membrane-bones is 

 primitively complete, as in Osteolepid fish, and several of them 



ppCU 



Fig. 131. — Osteolepis : dorsal view of a skull (in the collection of Prof. 

 D. M. S. Watson, F.R.S.), showing the course of the lateral-line canals. 

 Explanation of lettering for Figs. 131 to 149 : 



al, alisphenoid ; art, articular ; bo, basioccipital ; bpp, basipterygoid 

 process ; bs, basisphenoid ; c, canine ; d, dentary ; en, external nostril ; 

 eo, exoccipital ; ep, epipterygoid ; /, frontal ; fm, foramen magnum ; 

 i, incisor ; it, intertemporal ; /, jugal ; /, lachrymal ; Is, laterosphenoid ; 

 m, maxilla ; mp, mastoid process ; n, nasal ; 0, orbit ; oc, occipital con- 

 dyle ; on, otic notch ; 00, opisthotic ; op, opercular ; os, orbitosphenoid ; 

 p\, fourth premolar ; pa, parietal ; pe, periotic ; pf, postfrontal ; pi, pineal 

 foramen ; pi, palatine ; pm, premaxilla ; po, postorbital ; pop, preopercular ; 

 pp, paroccipital process; ppa, postparietal ; pr, prootic ; prf, prefrontal; 

 ps, parasphenoid ; psp, presphenoid ; pt, pterygoid ; pv, prevomer ; 

 q, quadrate ; qj, quadratojugal ; s, squamosal ; sm, septomaxilla ; sn, 

 spiracular notch ; so, supraoccipital ; st, supratemporal ; t, tabular ; 

 tb, tympanic bulla ; tp, transpalatine ; v, vomer. 



are traversed by the canals of the lateral-line system. In these 

 forms, the only openings in the roof of the skull are the orbits, 

 the chinks through which the spiracles opened, and the median 

 hole for the pineal eye which in fish is situated between the 

 frontal bones. In the higher bony fish or Teleostei, it is 



