HETEROMI 623 



the cranium in the latter type, the same may be said, in a 

 general way, of the skull. 



As pointed out by Emery, the very anterior position of the 

 vent in the Fierasferidae is directly related to the curious mode 

 of life of these Fishes, and the analogous condition obtained 

 in various families, such as the Gymnotidae, Xemichthyidae, 

 Amblyopsidae, shows it to be a character of relatively small 

 systematic importance. 



SYNOPSIS OF THE FAMILIES. 



A. Vent posterior. 



a. A distinct caudal fin ; ordinary scales small or wanting, but enlarged 



scutes along the side . . . . .1. Dercetidae f. 

 6. Tail tapering to a point ; scales cycloid. 



No spines ; dorsal fin short, anal very long . . .2. Halosauridae. 

 Fins with spines, dorsal short, anal long . . 3. Lipogenyidae. 



Dorsal fin formed of a series of spines, anal long, formed partly of spines 

 and partly of soft rays .... 4. Notacanthidae. 



B. Vent immediately behind the gill-opening ; no caudal fin ; scales absent 



5. Fierasferidae. 



Fam. 1. Dercetidae. Body much elongate; ordinary scales 

 small or wanting, but two or more continuous series of enlarged 

 scutes along each side ; mouth large, praemaxillaries apparently 

 forming the greater part of the upper border of the mouth, 

 which is toothed ; opercular apparatus complete. Dorsal fin 

 more or less extended, without spines ; anal short, caudal 

 separate ; ventrals with not less than 7 or 8 rays. 



Dercetis, Leptotrachelus, Leptecoclon, Pelargorhynclms, and 

 Stratodus, from the Upper Cretaceous of Europe, Syria, and 

 North America. 



Fam, 2. Halosauridae. Body elongate, covered with cycloid 

 scales, the tail tapering to a point, without caudal fin ; head 

 with scales ; mouth moderate, bordered by the praemaxillaries 

 and the maxillaries, both toothed ; suborbitals large ; praeopercle 

 rudimentary. Dorsal fin short, formed of soft rays, above or 

 a little behind the ventrals, which are rather far back, and 

 formed of 9 or 1 rays ; anal very long, without spines, extending 

 to the end of the tail. Ovaries transversely laminated, the 

 ova falling into the abdominal cavity. Some 10 living species 

 are known, referred to three genera, inhabiting the Atlantic, 

 Pacific, and Indian Oceans, at depths of 500 to 1400 fathoms. 



