PISCES. 431 



I. and II. are often grouped together as Chondropterygiis 



III. Ganoidei. Skeleton, cartilaginous or ossified ; investing bones always 

 present in relation with cranium and shoulder-girdle ; dermal skeleton forming large 

 scutes or scales ; ventral fins abdominal ; an operculum with opercular bones ; gill 

 filaments free; a spiracle sometimes present as in I; optic nerves forming a 

 chiasma ; a spiral valve in the intestine ; anus in front of a urogenital aperture ; 

 abdominal pores present ; an air-bladder with pneumatic duct ; a conus as in I ; 

 ova small, impregnated after exclusion. 



The living Ganoidei are (i) Chondrostei,vt\\h a heterocercal caudal fin ; naked 

 skin, partially covered with large and small bony scutes and a notochord not divided 

 into vertebrae. Atipenser, Scaphirhynchus, Spatularia (=Polyodori). (2) Holostean, 

 or bony Ganoidei, represented by three freshwater families, (i ) Polypteridae with two 

 genera, Polypterus and Calamoichthys, the former from West Africa and the Upper 

 Nile, the latter from Old Calabar. The pectoral fins are lobate ; the body covered 

 with rhombic scales, the caudal fin homocercal. (ii) Lepidosteidae represented by 

 Lepidosteus from North and Central America and Cuba, with fossil remains in 

 Europe. The fins have fulcra ; the caudal fin is slightly heterocercal ; the scales 

 rhomboid ; the vertebrae opisthocoelous. (iii) Amiadae represented by Amia calva 

 from North America. The scales are cycloid and large; the caudal fin 

 homocercal. 



There are many fossil forms. Accounts of them will be found in Giinther, 

 Study of Fishes, 1880, and in the decades of the English Geological Survey. 



IV. Teleostei (Osseous Fish). Skeleton well ossified ; a bony operculum with 

 branchiostegal rays ; investing bones of skull and shoulder-girdle well developed ; 

 caudal fin homocercal, rarely diphycercal ; optic nerves decussate ; spiral valve and 

 conus absent ; anus in front of genital and urinary apertures which may be separate 

 or form a urogenital cloaca. 



This order may be subdivided into 



1. Physostomi. Duct present to air-bladder ; ventral fins either absent (Apodes, 

 e. g. Muraena, Anguilla Gymnotus] or abdominal in position, e. g. Clupeidae^ Sal- 

 monidae, Cyprinidae, Siluroidei. Many are freshwater forms only. 



2. Anacanthini. Azygos and ventral fins with jointed fin-rays only ; the ventral 

 either jugular or thoracic ; air-bladder, when present, without a duct except in Lota 

 uulgaris, the Burbot or freshwater Cod. Gadoidei, Pleuronectoidei. Marine. 



3. Acanthopteri. Some of the rays of the dorsal, anal, and ventral fins are 

 spinous ; scales usually ctenoid ; no duct to air-bladder, e. g. Perddae, Gasterosteidae^ 

 Scomberidae, &c. Mostly marine. 



4. Pharyngognathi. Resemble 3, with which the group is often classified ; 

 but the two fifth branchial arches are fused into a single bone, e. g. Labridae or 

 Wrasses. Marine. 



5. Plectognathi. Globular or much compressed laterally; skin naked or fur- 

 nished with large scutes or spines ; ventral fins absent or represented by spines ; 

 praemaxillae and maxillae firmly united ; no duct to air-bladder, e. g. Ostracion^ 

 Diodon, Tetrodon. Marine. 



6. Lophobranchii. Long tubular snout with terminal edentulous mouth ; 

 dermal skeleton composed of large plates ; sub-opercular aperture very small ; gills 

 composed of processes arranged in tufts. Pegasidae ; Syngnathidae. 



2-6 are often grouped as Physoklisti. 



