stkuotuek of certain paleozoic dipnoi. 215 
Summary. 
This paper includes the first complete accounts of the circumorbital region 
and opercular apparatus of Dipterus valenneniiesi, and of the lower jaw of 
D. platyceplialus. It gives very nearly complete descriptions of the skull, 
lower jaw, and clavicular apparatus of Sagenoclus and Ctenodus which much 
extend our knowledge of these fish. In it the structures of the anterior end 
of the rare fish Uronemus and Concliopoma are described and figured for the 
first time. 
Dipiterus is shown to be directly comparable with Osteolepids in the 
structure of the opercular apparatus and the lower jaw, in addition to the 
many previously known resemblances. It is thereby shown that, as its early 
date would indicate, it is the most primitive known Dipnoan. 
Ctenodus and Sayenodus prove to be closely allied, and a detailed com- 
parison shows so great a similarity between the latter fish and Ceratodus as 
to leave no doubt that it is essentiallj^ ancestral to it. 
Uronemus and Concliopoma resemble one another only in the reduction in 
them of the tooth-plates to isolated denticles. In the structure of the palate 
and of the roof of the head they differ so much that they must represent 
widely-separated stocks. 
In the main, the trends of Dipnoan development suggested by Watson and 
Day are confirmed. It is, however, pointed out that the structure of the 
neural cranium of the Osteolepids, as described by Bryant in Eustlienopteron, 
is such that the Dipnoi cannot be direct descendants of that group, but that 
with it and the Amphibia they arose together from common ancestors at a 
time before the Middle Devonian. 
We have to thank Dr. A. Smith Woodward, of the British Museum, 
Dr. Kitchin, of the Museum of Practical Geology, Dr. Tattersall, of the 
Manchester Museum, and especially Drs. Eagle Clarke and J. Ritchie, of the 
Royal Scottish Museum, for the use of the materials in their care. 
List of Papers cited. 
1835. Agassiz, L. — Reoherches sur les Poissons Fossiles, vol. ii. 
1844. Poissons Fossiles des Vieux Gi'es Rouge. 
1898. DoLLo, L. — Sur la phylogeuie des Dipneustes. Bull. Soc. Beige de G^ol., vol. i.\. 
1889. Fbitsch, a. — Fauua der Gaskohle uud dei- Kalksteine der Permforiiiatioii Bohmeiis, 
Bd. ii. Heft 2, pp. 65-92, pis. 71-80. 
1909. Goodrich, E. S. — Vertebrata craniata, iu E. R. Laukeiter's ' Treatise on Zoology.' 
London, 1909. 
1915. Gebgoby, W. K. — Present status of the I'robleui of the origin of the Tetrapoda. 
New York Acad. Sci. Annals, vol. xxvi. 
1871. GtFNTHBB, A.- Description of Ceratodus. Phil. Trans. 1871, p. 586. 
LINN. JOUHN. ZOOLOGY, VOL. XXSV. 15 
