XXIII PLECTOGNATHI 723 
Halimochirurgus. 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 Spinacanthus, 
Eocene. 
Fam. 2. Triodontidae.—Praemaxillaries not protractile, 
firmly united to the maxillaries; teeth 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. No 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 Zriodon 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. 
Fic. 437.—Monacanthus, sp., with enlarged views of dorsal scales (4) and 
ventral spine (c). 
Gills 4. Praecaudal vertebrae with well-developed parapophyses, 
to which epipleurals are attached. Spinous dorsal fin with 1 to 
3 spines. Ventral fins, if present, represented by a single short 
