22 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



with Dipterus and Ctenodus, or with the Ganoids at all. Prof. Huxley l has lately 

 indicated a new distinction, which he considers of systematic importance, in the different 

 methods of suspension of the mandible. In some Ichthyopsida, ex. gr. Amphibia, 

 Chiincera, Marsipobranchii, and Sirenoidei, the mandible is connected with the skull by a 

 palato-quadrate suspensorium, which may be ossified or not, derived from the first 

 postoral (mandibular) arch. Hence such skulls are named ' autostglic.' In ' hyostylic ' 

 skulls, on the other hand (' Ganoidei, Teleostei, and ordinary Plagiostomes '), " the 

 palato-quadrate cartilage is no longer continuous with the chondrocraniuin (though the 

 bony elements of that arch may unite suturally with those of the skull, as in the Plecto- 

 gnathi), but is, at most, united with it by ligament. Moreover the dorsal element of the 

 hyoidean arch, or the hyomandibular, usually attains a large size, and becomes the chief 

 apparatus of suspension of the hinder end of the palato-quadrate cartilage with the 

 skull." 



Ceratodtis differs from the Ganoids also, according to Prof. Huxley, in the propor- 

 tions of the parts of the brain. We need not as yet insist upon this point. It differs 

 also from Lepidosiren ; and until the test of cerebral proportions has been more exten- 

 sively employed in the arrangement of fishes, we are ignorant of its real weight. 



It is more needful to observe that the Sirenoidei differ from Polypterus, the only 

 recent Crossopterygian Ganoid, in the absence of gular plates and in the presence of an 

 archipterygiurn, or jointed cartilaginous rod, articulated with the pectoral arch, and 

 giving off jointed radii with tolerable symmetry on both sides. Hence a line may be 

 drawn somewhere between the Sirenoidei and Polypterus ; and Prof. Huxley concludes 

 " that it is better to retain the Milllerian groups of Dipnoi {Sirenoidei, Muller), Ganoidei, 

 Teleostei, Plagiostomi, and Chimseroidei (Ilolocephali, Muller) as equivalent and distinct 

 natural assemblages." 2 



There are of course forms nearer to Ceratodtis and its allies than Polypterus. 

 Dipterus and Ctenodus are two of these ; and it is not easy to say whether the resem- 

 blances or the differences between the Dipnoi and the Ctenododipterini are of greater 

 weight. 



We do not know for certain that Dipterus and Ctenodus are hyostylic ; but the 

 suggestion has a certain degree of probability, and it may be worth while to offer some 

 remarks upon the classificatory value of this character. The carrying backwards of the 

 mandibular arch to the hyo- mandibular or uppermost piece of the hyoid arch serves, like 

 the loose connection of the palato-quadrate, to enlarge the gape. 3 It is an adaptive 



1 ' P. Z. S.,' 18/6, pp. 40 et seq. 



2 Loc. cit., p. 57. 



3 " In all Teleosteans, and in some to a most extreme degree, the gape of the mouth is placed at a 

 great distance from the head. In such fishes as the Dory (Zeus), in Epibulus, in Fistularia, and in the 

 ' Hippocampoids ' there is found the extreme of Teleostean modification of the mouth ; but in the most 

 moderate degree of specialisation, as in the Salmon, the hinge of the jaw is carried away from the head, first, 



