VIII SKULL 229 
sented by articular and mento-Meckelian elements. In its 
essential structure the skull of Zepidosteus' resembles that of 
Amia, but it has obviously undergone much specialisation. In 
some species (e.g. LZ. osseus) its appearance is greatly modified by 
the exceptional length and tapering shape of the beak, due to 
the elongation of that part of the skull which lies between the 
orbital and nasal regions; but in JL. platycephalus the reduced 
length and greater width of the beak, combined with its some- 
what flattened condition, impart an almost Crocodilian aspect 
to the head. Amongst other points of difference it may be 
mentioned that in Zepidosteus the continuity of the chondro- 
cranial roof is interrupted by a large superior fontanelle. There 
is no supra-occipital, and there are no lateral ethmoids, at all 
events in the usual position. The inter-orbital portion of the 
eranial cavity is largely obliterated by the formation of an inter- 
orbital septum, consisting of a thin vertical plate of bone, which 
either represents a pair of fused orbito-sphenoids or a pair of 
similarly modified lateral ethmoids. In addition to the ordinary 
investing dermal bones, including circum-orbitals, squamosal, and 
supra-temporals, there are numerous scale-like ossicles which 
take the place of the cheek-plates of Polypterus. The maxillae 
are segmented into numerous dentigerous bones fringing the 
margins of the upper jaw. The lower jaw has no mento-Meckelian 
bones, but there is a very complete series of dermal elements, 
including dentary, coronary, splenial, angular, and supra-angular 
bones in addition to an articular cartilage-bone. One of the 
most remarkable features in the skull of Zepidosteus is the exist- 
ence of a secondary articulation between the metapterygoid bones 
and a pair of transversely elongated condyles formed on each 
side by a lateral outgrowth from the parasphenoid and ali- 
sphenoid bones. By a horizontal sliding movement of the 
former on the latter, provision is made for the lateral expansion 
and contraction of the walls of the oral cavity and the separation 
and approximation of the lateral halves of the upper jaw.” 
The generality of Teleosts* more or less closely agree with 
Amia in the main features of their cranial structure. There are, 
however, certain minor features which are characteristic if not 
1 W. K. Parker, Phil. Trans. 173, 1882, p. 443. ? Bridge, P.Z.S. 1895, p. 302. 
3 Sagemehl, Morph. Jahrb. x. 1885, p. 1; xxvil. 1891, p. 489. Swinnerton, 
Quart. J. Micr, Sci. xly. 1902, p. 503. 
