XXIII 



PLECTOGXATHI 



723 



HalimocJiirurgus. The latter, remarkable for its long, tube-like 

 snout, is the only deep-sea form of this Sub-order ; it was recently 

 discovered in the Gulf of Manaar, at a depth of 143 fathoms. 

 Fossil genera are Acanthopleurus, Oligocene, and Spinacanthiis, 

 Eocene. 



Fam. 2. Triodontidae. Praemaxillaries not protractile, 

 firmly united to the maxillaries ; teetli coalescent into a beak, the 

 upper jaw divided by a median suture, the lower simple. Prae- 

 caudal vertebrae with or without parapophyses ; epipleurals 

 present. No spinous dorsal fin. Xo ventral fins. Abdomen 

 with a dilatable sac, kept expanded by the very long movable 

 pelvis. Body covered with small, spiny, subimbricate, bony 

 laminae. Vertebrae 20. 



A single species, the curious Triodon ~bursarius of the Indian 

 Ocean and Archipelago. 



Fam. 3. Balistidae. Praemaxillaries not protractile, firmly 

 united to the praemaxillaries ; teeth incisor-like; palatine mov- 

 ably articulated with ectopterygoid, or entirely free from it. 



FIG. 437. Monacanthus, sp., with enlarged views of dorsal scales (b) and 

 ventral spine (c). 



Gills 4. Praecaudal vertebrae with well-developed parapophysrs. 

 to which epipleurals are attached. Spinous dorsal fin with 1 to 

 3 spines. Ventral fins, if present, represented by a single short 



