TRANSACTIONS OF SECTION D. 777 



conclusion we are likely to draw is, that the older Dipnoan is a very specialised 

 form, that its heterocercal tail and separate dorsals and anal are due to specialisa- 

 tion from the continuous diphycercal dorso-ann-caudal arrangement iu the recent 

 forms, that the Holoptychiidaj were developed from it by shortening up of the 

 ventral archipterygium, as well as by the changes in cranial structure, and that 

 the Rhizodontidae and Osteolepidoe are a still more specialised series iu -which tlie 

 pectoral archipterygium has also shared the fate of the ventral in becoming 

 shortened up and uniserial. 



Five years ago, however, M. Dollo, of the Natural History Museum at 

 Brussels, the well-known describer of the fossil reptiles of Bernissart, proposed a 

 new view to the effect that the process of evolution had gone exactly in the 

 opposite direction ; ^ and after long consideration of the subject I find it difficult to 

 escape from the conclusion that this view is more in accordance with the facts of 

 the case, though, as we shall see, it also has its own difficulties. 



I have already indicated above that we are, on account of the more specialised 

 structure of the teeth, justified in considering the Holoptychians, with their 

 acutely lobate pectorals, a newer type than the Ehizodonts, even though they did 

 not survive so long in geological time. What, then, of the question of autostyly ? 



We do not know the suspensorium of HolopUjchius, but that of the Rhizodontidse 

 was certainly hyostylic, as in the recent Pohjpterus. Now as there can be no doubt 

 that the autostylic condition of skull is a specialisation on the hyostylic form, as seen 

 also in the Chimseroids and in the Amphibia, to suppose that the hyostylic 

 Crossopterygii were evolved from the autostylic Dipnoi is, to say the least, highly 

 improbable ; in my own opinion, as well as in that of M. Dollo', it will not stand. 

 And if we assume a genetic connection between the two groups it is in accordance 

 with all analogy to look on the Dipnoi as the children and not as the parents of 

 the Crossopterygii. 



_ M. Dollo adopts the opinion of Messrs. Balfour and Parker that the apparently 

 primitive diphycercal form of tail of the recent Dipnoi is secondary, and caused 

 by the abortion of the termination of the vertebral axis as in various ' Teleostei,' 

 60 that no argument can be based on the supposition that it represents the original 

 ' protocercal ' or preheterocercal stage. Very likely that is so, but it is not of so 

 much importance for the present inquiry, as both in the Osteolepidae and Rhizodon - 

 tidae we find among otherwise closely allied genera some which are heterocercal, 

 others more or less diphycercal. Biplopterus, for example, differs from Thursius 

 only by its diphycercal tail, and in like manner among the Rhizodontidffi Tristi- 

 chopterus is heterocercal, Eusthenoptero7i is nearly diphycercal, and there can be no 

 doubt that, in spite of this, their caudal fins are perfectly homologous structures. 



But of special interest is the question of the primitive or non-primitive nature 

 of the continuity of the median fins in the recent Dipnoi. Like others I was 

 inclined to believe it primitive, and that the broken-up condition of these fins in 

 Dipterus was a subsequent specialisation, and in fact gave the series Phayieropleuron 

 Scaumenacia, Diptenis macroptenis, and D. Valenciennesii as illustrating this 

 process of differentiation.- This view of course draws on the imperfection of the 

 geological record in assuming the existence of ancient pre-Dipterian Dipnoi with 

 continuous median fins, which have never yet been discovered. But Dollo, using 

 the very same series of forms, showed good reason for reading it in exactly the 

 opposite direction. 



The series is as follows : — 



1. Dipterus Valenciennesii Sedgw. and Murch., from the Orcadian Old Red, 

 and the oldest Dipnoan with whose shape we are acquainted, has two dorsal fins 

 with short bases, a heterocercal caudal, and one short-based anal. 



2. Dipterus macropterus Traq., from a somewhat higher horizon in the 

 Orcadian series, has the base of the second dorsal much extended, the other fins 

 remaining as before. 



' ' Sur la Phylog6nie des Dipneustes,' Bulletin Soc. beige geol. paleont. TiydT 

 vol. ix. 189i". ¥ * ;> '> 



? Geol, Mag. (3), vol. x. Ig93, p. 2Q3. 



