110 PISCES. 



raised into a rounded ridge to meet the deepened trunk. The cheek is 

 covered by two large suborbitals (postorbitals) behind, apparently one 

 preorbital, and a discontinuous series of relatively small circumorbitals. 

 The sclerotic is ossified. The premaxilla on each side seems to be fused 

 with the ethmoid; the maxilla is long and slender, with a small supra- 

 maxilla almost covering its posterior end. The dentary bone of the 

 mandible is excessively large, the angular very small; and there is a 

 thin splenial entering the symphysis. The marginal teeth above and 

 below occur in a regular series, and none are much enlarged. The 

 preoperculum is a wide triangular plate; the operculum is narrowed 

 above and the suboperculum is relatively very large ; the interoperculum 

 is small. The number of branchiostegal rays varies from 40 to 50. No 

 ossifications have been observed in the sheath of the notochord, but the 

 vertebral arches are at least superficially ossified. The bases of the 

 arches are narrow and deep ; the neural spines are free in the abdominal 

 region, but both neural and hajmal spines are fused with their respective 

 arches in the caudal region; the ribs are especially delicate and are far 

 from reaching the ventral border. A single haemal spine at the base 

 of the caudal fin is enlarged into a broad triangular expansion for the 

 support of this unusually powerful appendage. So far as known, Pachy- 

 cormus is confined to the Upper Lias, on which horizon it occurs in 

 England, Wiirtemberg, Bavaria, and France. The typical species is P. 

 macropterus, which attains the length of not less than one metre. 



FIG. 78. 



Hypsocormus insignis ; restoration, scales omitted, about one-eighth nat. size. 

 U. Jurassic (Lithographic Stone) ; Bavaria. (From Brit. Mus. Catal.) 



Hypsoconnus (fig. 78). A larger fish than Pachycormus, often with 

 longer rostrum, free triangular premaxilla, stouter splenial entering the 

 mandibular symphysis, larger teeth, small pelvic fins, and much-extended 

 anal fin. The parasphenoid bears a patch of minute granular teeth; 

 the vomers are fused together and with the ethmoidal rostrum, bearing 

 among other teeth one very large pair of tusks. The pterygo-palatine 

 arcade exhibits moderately large teeth on the edge, becoming granular 



