432 THE ANIMAL KINGDOM. 



V. Dipnoi. Body covered with scales ; caudal fin diphycercal ; paired limbs 

 with a central axis ; with branchiae and lungs ; persistent notochord without verte- 

 bral centra ; a spiral valve in intestine and a cloaca ; abdominal pores ; a conus of 

 large size spirally twisted. The order contains two sub-orders. 



(1) Monopneumona. With single lung; large cycloid scales; paired limbs with 

 bilateral rays. Ceratodus from rivers of Queensland. 



(2) Dipneumona. Lung double ; paired fins linear. Protopterus annectens 

 and P. amphibius from tropical Africa, with five gill-clefts and a lateral membrane 

 to the paired fins, and Lepidosiren paradoxa from Brazil, with four gill-clefts and no 

 lateral membrane to the paired fins. 



Elasmobranchii. See p. 279. 



Holocephali. Leydig, Chimaera, Archiv fur Anat. und Physiol. 1851. 



Ganoidei. Skull of Adpenser : structure and development, W. K. Parker, 

 Ph. Tr. 173, 1882; Skull of Lepidosteus, Id. ibid. Structure and development of 

 Lepidosteus, Balfour and W. N. Parker, ibid, with lit. cited. External gills of 

 Polypterus, Steindacher, Hyrtl. SB. Akad. Wien, Ix. Abth. i. 1869. 



Teleostei, p. 89-90, 98, 102. Augendhnliche Organe der Fische, Leydig, Bonn. 

 1881; Macchie splendenti of Scopelus, Emery, Mitth. Zool. Stat Naples, v. 1884. 

 Leptocephalidae, Kolliker, Z. W. Z. iv. 1852 ; Carus, Ueber die Leptocephalidae, 

 Leipzig, 1 86 1. Eyes of Pleuronectidae, Agassiz, 'Young Stages,' &c., Proc. Amer. 

 Acad. xiv. 1879. 



Sexual organs of Muraenoids, Brock, Mitth. Zool. Stat. Naples, ii. 1881. 

 Viviparous Teleostei, Ryder, Proc. U. S. Nat. Museum, viii. 1885. Relations of 

 Yolk to Gastrula, Cunningham, Q. J. M. xxvi. Pelagic stages of Young Fish, 

 Agassiz and Whitman, Mem. Harvard Mus. xiv. (i), 1885. 



Dipnoi, Ayers, J. Z. xviii. 1885, with lit. cited. Fins of Dipnoi and species of 

 Protopterus, Schneider, Z. A. ix. 1886. Ceratodus, Giinther, Ph. Tr. 161, 1871; 

 Huxley, P. Z. S. 1876. Nostrils in Lepidosiren, Huxley, ibid. Skeleton and nervous 

 system of Protopterus, Wiedersheim, J. Z. xiv. 1880 ; Brain, Fulliquet, Recueil Zool. 

 Suisse, iii. 1886. Heart, branchial arteries (also in bony Ganoidei), Boas, M. J. vi. 

 1880. Ovarian ovum of Protopterus, Beddard, Z. A. ix. 1886. 



CLASS CYCLOSTOMI (Marsipobranchii). 



Elongated Eel-like Ichthyopsida, with the mouth not supported by jaws, 

 as in other Vertebrata; with small azygos fins, but devoid of paired fins^; no 

 exo-skeleton ; with a single nostril, and six or seven pairs of branchial 

 pouches. 



The azygos fin is small and confined to the tail in the Myxinoidei, but 

 extends forwards continuously along the back in the larval Lamprey or 

 Ammocoetes, whereas in the adult or Petromyzon the antero-dorsal portion 

 is separated off as a dorsal fin. 



The superficial cells of the epidermis in the Lampreys (Petrowyzon- 



1 Dohm believes that the two ridges, one on either side the anus in Petromyzon, represent rem- 

 nants of pelvic fins. See Mitth. Zool. Stat. Naples, vi. 1885. 



