ACTINOPTERYGII. Ill 



on the inner face. The splcnial in its hinder portion is a thin deep 

 plate covered with granular teeth ; but its symphysial portion bears 

 large teeth to oppose the vomerine pair. The large teeth on the dentary 

 are in a single spaced series, those at the syrnphysis pointing a little 

 forwards. All the teeth are round or oval in transverse section, and a 

 slight vertical striation is due to their complex structure. They consist 

 of a bundle of minute tubes of vascular dentine, seen as well-defined 

 areas in a transverse section under a low power of the microscope. The 

 typical species is ffypsocormus insignia (fig. 78), well preserved in the 

 Lithographic Stone (Lower Kimmeridgian) of Bavaria, and it is asso- 

 ciated with a still larger species, H. macrodus. Closely related to the 

 latter are H. leedsi and H. tenuirostris from the Oxford Clay of Peter- 

 borough, known only by fragmentary specimens which are of importance 

 as exhibiting many features in the osteology of the genus. 



Protosphyrsena. A still larger fish than Hypsocormus, nearly always 

 with a longer rostrum, but as yet known only by the head, pectoral fins, 

 and other fragments. Distinguished from Hypsocormus, so far as known, 

 merely by the lateral compression of the teeth, which have opposite sharp 

 edges, and by these teeth being fixed in complete sockets. A charac- 

 teristically Upper Cretaceous genus, the type species being Protosphy- 

 rcena ferox from the English Chalk. Fragments of several species occur 

 in the Cambridge Greensand. Nearly complete skulls and fine pectoral 

 fins have been found in the Chalk of Kansas, U.S.A. 



Sub-Order 3. Aetheospondyli. 



Here are provisionally placed some highly-specialized ganoids 

 of uncertain relationships. So far as their skeleton is concerned, 

 they differ from the Protospondyli merely in the nature of the 

 vertebrae, which are not known to exhibit even in their most 

 immature stages any indication of separate pleurocentra and 

 hypocentra. In one family (Aspidorhynchidoe) the centra are 

 ring-shaped or complete and biconcave ; in the other family 

 (Lepidosteidae) the centra are unique among fishes in being 

 opisthoccelous. 



The Aspidorhynchidae range from the Lower Oolites to 

 the Upper Chalk inclusive. They are elongated fishes with pro- 

 duced rostrum, rhombic scales much deepened on the flank, all 

 the fins small, and the dorsal and anal fins remote. The fin- 

 rays are stout, and fulcra are minute or absent. The mandible 

 is remarkable as exhibiting a bilaterally-symmetrical median 

 piece (predentary bone) in front of the usual elements. 



