vm SKULL 229 



sented by articular and mento-Meckelian elements. In its 

 essential structure the skull of Lepidosteus ^ resembles that of 

 Amia, but it has obviously undergone much specialisation. In 

 some species (e.g. L. 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 L. 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 Lepidosteus 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 

 cranial 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 PolyxJterus. 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 Lepidosteus 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, PHI. Trans. 173, 1882, p. 443. » Bridge, P.Z.S. 1895, p. 30;^. 

 5 Sagemeh), Morph. Jahrb. x. 1885, p. 1 ; xxvii. 1891, p. 489. Swinnerton, 

 Quart. J. Micr. Sei. xlv. 1902, p. 603. 



