4i6 



TELEOSTEl 



TRIBE 4. HYPOSTOMIDES (Incertae sedis). 



A small group of highly specialised fish the systematic position of 

 which is very uncertain, although they are usually associated with the 

 Lophobranchs. 



The body is short and depressed; the head produced into a long 

 snout beyond the mouth, which is small, ventral, and toothless. Rings 



FIG. 416. 

 Pegasus natans, L. (After Giinther.) 



of scutes cover the body. The gill-opening is very small. The enlarged, 

 horizontal pectoral fins bear five spinous dermotrichs. Pelvic fins are 

 represented by a few dermal rays behind the pectoral. There are no 

 ribs, no air-bladder, no symplectic, and no preopercular. 



Family PEGASIDAE. Pegasus, L. (Fig. 416) ; Indo-Pacific. 



Series 5. 

 Sub-Order 5. NOTACANTHIFORMES (Heteromi). 



This is a group of highly specialised, mostly deep-sea fish, of 

 somewhat doubtful affinity, to which have been added the extinct 

 Dercetidae. In the modern forms the caudal region is relatively 

 long, with a tapering extremity from which the caudal fin has dis- 

 appeared. A snout is usually present (especially in the Halosauridae 

 and Notacanthidae). The pelvic fins are abdominal, and the median 

 fins may be spiny. The duct of the air-bladder is closed. The 

 parietals meet in a median suture, and the pectoral girdle is joined 

 to the skull by a reduced simple post-temporal, attached to the 

 supraoccipital, or by a mere ligament. When present, the scales 

 cover the head as well as the body. 



This sub -order was founded by Boulenger [42] to include 

 the Lyopomi and Heteromi of Gill, highly specialised deep-sea fish, 

 and a few other doubtfully related genera. 



