NOTACANTHIFORMES 417 



DIVISION 1. 



There are separate dorsal, anal, and caudal fins. 



Family DERCETIDAE. An extinct i'amily of fish with elongate body, 

 extended dorsal and short anal fin, seven or eight dermal rays in the 

 pelvic fin, and no spines. The scales are vestigial, either very small or 

 absent ; but a row of large scutes extends along each side. There is a 

 snout, a large mouth bordered by the premaxillae, and small teeth. 

 These Cretaceous fossils possibly represent a stage in phylogeny before 

 the caudal fin was lost. 



Dercetis, Ag. ; Europe. Leptotrachelus, van der M. ; Europe and 

 Asia. Pelargorhynchus, van der M. ; Stratodus, Cope North America. 



DIVISION 2. 



The tail is pointed, and the caudal fin has been lost, while the 

 anal has become very long. The pectoral fins are generally inserted 

 high up, especially in the Notacanthidae and Halosauridae. 



TRIBE 1. 



There are cycloid scales, and the anus is normally situated. 



Family HALOSAURIDAE. The border of the small mouth is formed 

 by the premaxillae and maxillae, both provided with minute teeth. The 

 preopercular is vestigial. All the dermal rays are soft, the dorsal fin is 

 short. A canal for the notochord pierces the centra. The ova are shed 

 freely into the coelom. Halosauropsis has phosphorescent organs along 

 a series of enlarged lateral scales. 



Halosauropsis nigerrimiis, showing the phosphorescent organs lodged in the large 

 scales of the lateral line. (After Alcock, No.turalist in Indian Seas.) 



Echidnocephalus, van der .M. ; Enchelurus, van der M. Cretaceous, 

 Europe and Mount Lebanon. Halosaurus, Johnson ; Halosauropsis, Coll. 

 deep-sea (Fig. 417). 



Family LIPOGENYIDAE. The genus Lipogenys differs from the pre- 

 ceding family chiefly in the possession of spines in the anal fin and three 

 spines in the pelvic fin, and in the modification of the mouth, which is 

 small, suctorial, and ventral. The jaws are toothless and specialised. 

 Small scales cover the body. 



Lipogenys, G. and B. ; North Atlantic (Fig. 418). 



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