SUB-CLASS IV DIPNOI 61 



clasps the head shield on each side. H. asmussi, Ag. sp., common in Upper 

 Devonian of Livonia, but known only by the massive detached plates. 



Aspidichthys, Glyptaspis, Trachosteus, Newberry; Anomalichthys, v. Koenen ; 

 Asteroplax, Sm. Woodw. Devonian. 



Mylostoma, Diplognathus, Newberry; Selenosteus, Stenosteus, Dean. Upper 

 Devonian (Cleveland Shale) ; Ohio. 



Most of the genera enumerated on this and the preceding page are 

 regarded by Dean and others as types of distinct families. 



Sub-Class 4. DIPNOI, Mtiller. 1 Lung-fishes. 



Skeleton partially ossified, with numerous well-developed membrane bones. Pterygo- 

 quadrate arcade completely and immovably fused with the cranium; gill clefts feebly 

 separated, opening into a cavity covered with a bony operculum. Paired fins paddle- 

 like, with a long, segmented cartilaginous axis (archipterygium of Gegenbaur) ; tail 

 diphycercal or heterocercal. In the living forms — optic nerves not decussating, but 

 forming a chiasma, bulbus arteriosus of the heart with numerous valves, intestine with 

 a spired valve, and air-bladder lung-like. 



The internal skeleton of the Dipnoi is chiefly cartilaginous, but the upper 

 and lower vertebral arches, the ribs and fin-supports, all exhibit some tendency 

 towards ossification. 



The Dipnoi differ so much from all fishes, in the modification of the air- 

 bladder into a single or double elongated sac with numerous cellular spaces, 

 which serves as 'a lung and is connected by a short tube with the anterior 

 wall of the gullet ; moreover, in the peculiar characters in the structure of 

 the heart, in the presence of internal narial openings, and in the possession 

 of the faculty of existing for a considerable period out of water, that they 

 have often been regarded as fish-like Amphibia or scaly Sirens. The discovery 

 of the "Barramunda" (Ceratodus forsteri) in the rivers of Queensland confirmed 

 the idea of their relationship to the Palaeozoic Crossopterygians previously 

 suggested by Huxley. Nevertheless, they are distinguished from these and 

 from all other Ganoids and Teleosteans by the autostylic arrangement of the 

 jaws. They are divided into the two orders of Ctenodipterini and Sirenoidei. 



Order 1. CTENODIPTERINI. Pander. 



Cranial roof bones snail and numerous. Pays of median fins very fine, much 

 more numerous than their supports, which are directly apposed to the vertebral arches. 



Family 1. Uronemidae. Traquair. 



Upper dentition comprising a cluster of small, blunt, conical denticles on the 

 palatine bones; lower dentition of similar denticles on the splenial. Median fins 

 continuous and tail diphycercal. Lower Carboniferous to Lower Permian. 



1 Gtuither, A., Description of Ceratodus (Phil. Trans, vol. CLXI.), 1871-72.— Huxley, T. 1L, 

 Proc. Zool. Soc. 1876, p. 24. — Miall, L. C, Monograph of the Sirenoid and Crossopterygian Ganoids 

 (Palaeont. Soc), 1878. — Pander, C. H., Ueber die Ctenodipterinen, etc., des devonisehen Systems. 

 St. Petersburg, 1858. — Traquair, II. H., On the genera Dipterus, Palaedaphus, Holodus, and 

 Gheirodus (Ann. Mag. Xat. Hist. ser. 4, vol. XVII.), 1878 (also ibid, ser. 5, vol. II.).— Teller, F., 

 Ueber Ceratodus sturi (Abh. k.k. geol. Reichsanst. Wien. vol. XV.), 1891. — Zittel, K. A. >■<,,,. Ueber 

 Ceratodus (Sitznngsb. k. bay. Akad. YYiss.. math.-phys. CI.), 1886. 



