PISCES 291 



by thin calcareous rings in the chorda membrane. There is only one 

 pair of external openings of the gill clefts. 



631. Order 3. — The Dipnoi or lung fishes. In this group there are four 

 pairs of gills which are covered by an operculum. The tail is diphycercal 

 i. e., the tail fin is symmetrical around the straight spinal axis. The 

 chorda persists. The skeleton is cartilaginous and partly bony. There 

 is a spiral valve and a pair of lungs. These fishes live in tropical regions 

 in rivers and ponds which dry up in the dry season. During the dry season 

 the animals bury themselves in the mud. 



632. Order 4. — The Brachioganoidea also have gills covered by an 

 operculum and a diphycercal tail. The skeleton is bony. The body is 

 covered with thick rhombic scales covered with ganoin, a kind of enamel. 

 There is also a spiral valve. 



633. Order 5. — The Chondroganoidea or sturgeons have the gills covered 

 by an operculum; a persistent chorda and cartilaginous skeleton; the head 

 prolonged into a snout; the skin is naked or with bony plates; the tail 

 fin heterocercal, i. e., the axis is bent up and the fin is unsymmetrical; 

 a spiral valve and a conus arteriosus. The number of genera is small. 

 The fishes are found chiefly in fresh waters. 



634. Order 6. — The Rhomhoganoidea or gar pikes have gills covered by 

 an operculum; a bony skeleton; a long snout; body covered with rhombic 

 ganoid scales; tail heterocercal; rudimentary spiral valVe and a conus 

 arteriosus. A few species only; found in the rivers and lakes of North 

 America. 



635. Order 7. — The Cycloganoidea have operculum, bony skeleton, 

 cycloid scales, tail heterocercal, rudimentary spiral valve and a conus 

 arteriosus. There is only one species, Amiatus calvus. This is found in 

 the streams of North America. 



636. Order 8. — The Teleostei or true bony fishes are very numerous. 

 They have an operculum, bony skeleton, ctenoid or cycloid scales or 

 large bony plates, a bulbus arteriosus, no spiral valve. The order is 

 divisible into twelve sub-orders with many families. Some of the families 

 are the herrings, salmon, electric eel, carp, catfish, eels, pike, sea horse, 

 mullet, perch, mackerel, flat fishes, toad fish, trunk fish, etc. 



637. Class m. Amphibia. — In this group of animals the 

 larva is typically aquatic, the adult terrestrial. This is pri- 

 marily evident in the respiratory organs though in many cases 

 a marked metamorphosis occurs which involves other organs. 

 The Amphibia have two pairs of pentadactyl appendages, a 



