CHAPTER VII. 



SUB-CLASS GANOIDEI. * 



Fishes with a conus arteriosus, optic chiasma, free pectinate gills 

 and an operculum, abdominal pelvic fins, a spiral valve in the intes- 

 tine, an air-bladder, and without a processus falciformis and 

 choroid gland. The oviducts and urinary ducts unite and open by 

 a common urogenital aperture behind the anus. The skull is hyo- 

 stylic and is without a supraoccipital bone. The segmentation of 

 the ovum is complete, a pronephros is present in the larva and 

 abdominal pores are always found. 



Very few of the characters mentioned in the definition are pecu- 

 liar to Ganoids : they are almost all found in Teleostei or Elas- 

 mobranchii. This fact coupled with the great variations of 

 structure found in the group points to the conclusion that the 

 sub-classes, Elasmobranchii, Teleostei, Ganoidei and Dipnoi are 

 the survival of a once great and continuous group of animals, a 

 large number of which have become extinct, leaving three groups, 

 Elasmobranchii, Teleostei and Dipnoi, each fairly compact and 

 showing but little variety of organization, and one, the Ganoidei r 

 loose and heterogeneous with large gaps between the individual 

 members. Although it may fairly be held that by such forms 

 as Lepidosteus and Amia the Ganoids more nearly approach the 

 Teleostei than the Chondrostei do the Elasmobranchii ; we cannot 



* J. Miiller, Ueb. d. Bau u. d. Grenzen der Ganoiden, Abh. d. Berlin 

 Akad. d. Wiss. 1844; id. Myxinoiden, op.cit. J. Hyrtl, Ueb. den Harn- 

 werkzeuge bei den Ganoiden, Wien Denkschriften, 8, 1855. Liitken, Ueb. d. 

 Begrenzung und Eintheilung d. Ganoiden, Palaeontographica, 22, 1872, 

 Huxley, ' The systematic arrangement of the fishes of the Devonian. 

 Epoch,' Mem. Geolog. Survey. London, 10, 1861 ; and 12, 1866. Tra- 

 quair, Ganoid fishes of the British Carboniferous Formations, Palaeontogr. 

 Soc. 1877. V. Wijhe, Visceral Skel. u. Nerven des Kopfes der Ganoiden u. 

 Ceratodus, Nied. Arch. Zool., 5, 1882, p. 207. Zittel, Grundzuge d. Palaeon- 

 tologie, Leipzig, 1895 (English Translation, Macmillan, 1902). Smith- 

 Woodward, Outlines of Vert. Palaeontology, Cambridge, 1898. 



