254 FOSSIL FISHES OF THE ENGLISH CHALK. 



linked with the highest Jurassic fishes by the Scopeloids, which occur throughout 

 Cretaceous formations in great abundance and variety. Next to the Elopines and 

 Clupeoids, in fact, the Scopeloids are the most characteristic fishes of the Creta- 

 ceous fauna. Some of them are almost identical with genera which still live in 

 the deep sea, but nearly all have the bones so stout and well calcified that they 

 must have been surface-dwellers. They, like many other Cretaceous types of fishes, 

 have, therefore, probably migrated to the ocean depths during the Tertiary period 

 as the competition from newer types of fishes has increased. 1 



The Cretaceous Dercetidae are also primitive teleosteans whose surviving 

 relatives live in the deep sea. They, again, have a well-hardened skeleton, but it is 

 interesting to note that one known specimen exhibits traces of the distensible 

 stomach which is so common a feature in abyssal types (see p. G9). They seem to 

 have become extinct at the end of the Cretaceous period, but the Llalosauridae, 

 which lived with them, are still well represented in the ocean depths, and the 

 skeleton of the typical existing genus Halosaurus can scarcely be distinguished 

 from that of Echidnocephalus found in the Upper Senonian of Westphalia. 



The gradations between the Cretaceous groups already mentioned and the 

 highest Jurassic fishes are not difficult to perceive; but the connection between 

 the Apodes, or eels, and their predecessors, is not yet evident. That these fishes 

 are the degenerate latest members of an old group is shown, among other features, 

 by their long-bodied shape, the continuous extension of their median fins, and the 

 absence of pelvic fins; but the L T pper Cretaceous Urenchelys? only makes a slight 

 approach towards its normally fish-shaped ancestors in still exhibiting a separate 

 caudal fin, while the contemporaneous Anguillavus 3 merely adds to this primitive 

 character the retention of the pelvic fins. Below the Upper Cretaceous no fishes 

 have hitherto been discovered tracing the line of eels further back; and the fact that 

 some of the generalised forms, even in the existing fauna, have more than five basal 

 bones in their pectoral fins, suggests that the direct ancestors are to be found, not 

 amonp- the so-called teleosteans, but in some of the Jurassic " ganoids." It might 

 be profitable to make a detailed study of the Macrosemiidae in this connection. 



The Cretaceous Clupeoids are chiefly of interest on account of their precocious 

 development. They do not differ much from some of the Jurassic Leptolepidse, 

 but it is remarkable that so far back as the Lower Cretaceous, both in Switzerland 4 



1 A. S. Woodward, " The Antiquity of the Deep-Sea Fish-Fauna," Natural Science, vol. xii (1898), 

 pp. 257—260, pi. x. 



2 See especially A. S. Woodward, Catal. Foss. Fishes Brit. Mus., pt. iv (1901), pp. 337—339, pi. 

 xviii, figs. 1 — 3. 



3 O. P. Hay, " On a Collection of Upper Cretaceous Fishes from Mount Lebanon, Syria," Bull. 

 Amer. Mus. Nat. Hist., vol. xix (1903), pp. 437 — 441, pi. xxxvi, figs. 2, 3, pi. xxxvii, fig. 1. 



1 Chqjea antiqua and C. voirone/isis, F. J. Pictet, "Foss. Terrain Neocom. Voirons " (Palcont. 

 Suisse, 1858), pp. 31, 37, pi. iv, figs. 7—13, pi. v, figs. 1—10. 



