DIPNOI. 



267 



five angled box composed of polygonal body plates, firmly united. Ostracion 

 Lactophrys, trunk-fishes. Ostracion first appear .in the eocene. GYMNO- 

 DONTI, spinous dorsal lacking ; scales spiniform or absent ; jaws with 

 enamelled plates, but without distinct teeth. Tetrodon, Dtodon, Chilomyc- 

 terus, etc., swell-fishes or globe-fishes, etc., the common names arising 

 from the powers of inflation possessed by them. Mola (Orthagoriscus}, 

 sun-fish, the most bizarre of fishes, seemingly but a large head with fins 

 attached. Fossils doubtfully referred to Mola occur in the British creta- 

 ceous ; Dtodon appears in the eocene. 



FIG. 268. Sun-fish, Mola rotunda, after Putnam. 



SUB-CLASS III. DIPNOI (DIPNEUSTES). 



Fishes with partially ossified cartilage, numerous membrane 

 bones, and persistent notochord ; skull autostylic ; a membra- 

 nous operculum ; tail diphycercal ; paired fins archiptergial or 

 reduced ; heart with multivalvular conus, a spiral valve in in- 



