142 WEALDEN AND PURBECK FOSSIL FISHES. 



Among the Selachians, the well-preserved skulls of Hybodus basanus are 

 especially interesting, because they exhibit much more resemblance to those of 

 the Notidanidse than to the skull of the modern Cestracion, to which Hybodus is 

 commonly regarded as nearly allied. The teeth of some of the Lower Jurassic 

 species of Hybodus, indeed, seem to pass into those of the earliest known species 

 of Notidanus 1 ; just as the high-cusped teeth of the Wealden H. basanus are closely 

 similar to some of the Middle and Upper Jurassic teeth of Orthacodus* which seem 

 to pass into those of the primitive though typical Lamnidse of the Cretaceous 

 period. Some of the more generalised Hybodonts, when better known, may 

 therefore prove to be ancestral to several later t} r pes of sharks. 



Hybodus ha sunns is also noteworthy in having only one pair of hooked cephalic 

 spines instead of the two pairs, of nearly equal size, which characterise the early 

 Liassic species. 3 In one species of the allied genus Aster acanthus, from the Oxford 

 Clay of Peterborough, the late Mr. Alfred N. Leeds observed that of the two pairs 

 of cephalic spines one was much smaller than the other. There was thus probably 

 a similar reduction of one pair in some species of Hybodus, which eventually 

 resulted in its complete loss. 



The occurrence of a dwarf species of Acrodus in the Wealden is paralleled by 

 that of a similar dwarf species in an estuarine deposit of nearly the same age in 

 Bahia, Brazil. ' This is almost the last appearance of the genus, the latest known 

 species being Acrodus levis from the Gault. 



No remains of Crossopterygians have hitherto been found in the Wealden, and 

 the only specimen from the Purbeck Beds is that of the typically Jurassic I' ml inn 

 now described. The ornament of the scales clearly distinguishes it from the 

 Cretaceous Macropoma. 



Among Chondrosteans the Palgeoniscidas are represented for the last time by 

 the genus Goccolepis, which is too specialised to be ancestral to the later sturgeons. 



The representatives of Lepidotus are interesting as including a comparatively 



generalised species, L. minor, which might even be Lower Jurassic, besides a large 



.and highly specialised species, L. mantelli, which could not be earlier than Upper 



Jurassic, and might be Lower Cretaceous. The vertebral ossifications in the latter 



species are particularly remarkable. 



The Pycnodonts also comprise both generalised and moderately specialised 



1 Compare Hybodus polyprion, Ag., with Notidanus muensteri, Ag. (A. S. Woodward, Geol. 

 Mag., [3] vol. iii (1886), p. 257, pi. vi, figs. 1—5). 



2 O. Jaekel, Sitzuugsb. Gesell. naturf. Freuude, Berlin, 1898, p. 139, text-fig. 2; F. Priem, 

 Bull. Soc. Geol. France, [4] vol. xii (1912), p. 254, with figs. A large tooth apparently of 

 Orthacodus, from the Danian of Scandinavia, has been described as Ozyrhina lundgreni, J. W. Davis, 

 Trans. Eoy. Dublin Soc, [2] vol. iv (1890), p. 393, pi. xxxix, figs. 8—13. 



3 See Hybodus delabechei (Charlesworth), A. S. Woodward, Catal. Foss. Fishes, Brit. Mus., 

 pt. i (1889), P . 259, pi. viii, fig. 1. 



* Acrodus nitidus, A. S. Woodward, up. at. (1889), p. 297, pi. xiv, fig. 8. 



