Notices oj Memoirs — Br. R. H. Tmquair's Address. 519 



a dentigerous maxilla, the skull is autostylic, and the palatojjterygoids 

 and the mandibular splenial are like those of Ceratodus and bear 

 each a tooth-plate with radiating ridges. 



Now, comparing Dipterus with the recent Ceratodus and Protop- 

 terus, the first conclusion we are likely to draw is, that the older 

 Dipnoan is a very specialized form, that its heterocercal tail and 

 separate dorsals and anal are due to specialization from the con- 

 tinuous diphycercal dorso-ano-caudal arrangement in the recent forms, 

 that the Holoptychiidae were developed from it by shortening up of 

 the ventral archipterygium, as well as by the changes in cranial 

 structure, and that the EhizodontidEs and Osteolepidee are a still 

 more specialized series in which the pectoral archipter3^gium has 

 also shared the fate of the ventral in becoming shortened up and 

 uniserial. 



Five years ago, however, M. Dollo 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 

 difticult 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 specialized 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 HoloptijcMus, but that of the 

 Rhizodontidte was certainly hyostylic, as in the recent Pohjpterus. 

 Now, as there can be no doubt that the autostylic condition of skull 

 is a specialization on the hyostylic form, as seen also in the 

 Chimeeroids 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 con- 

 nection 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,' so 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 Osteolepidfe 

 and Ehizodontidce we find among otherwise closely allied genera 

 some which are heterocercal, others more or less diphycercal. 

 Diplopterus, for example, differs from Thursius only by its diphy- 

 cercal tail, and in like manner among the Rhizodontidae Tristi- 

 chopterus is heterocercal, Eusthenopleron is nearly diphycercal, and 



' " Sur la Phylogeuie des Dipneustes " : Bull. Soc. beige geol. paleont. hydr.' 

 vol. ix (1895). 



