778 EEPORT— 1900. 



3. In f'caumenacia curta (Whiteaves), from the Upper Devonian of Canada, 

 tLe first dorsal has advanced considerably towards the head, and its base has now 

 become elongated, while the second has become still larger and more extended, 

 though still distinct from the caudal posteriorly. 



4. In Thaneropleurun Andcrsoni Huxley, from the Upper Old Eed of Fife- 

 shire, the two dorsal fins are now fused with each other and with the caudal, 

 forming- a long continuous tin along the dorsal margin, while the tail has become 

 nearly diphycercal, with elongation of the base of the lower division of the fin. 

 But the anal still remains separate, narrow, and short-based. 



5. In the Carboniferous Uronemus lobatus Ag. the anal is now also assorted 

 in the lower division of the caudal, forming now, likewise on the haemal aspect, a 

 continuous median fin beliind the ventrals. There is also a last and feeble remnant 

 of a tendency to an upward direction of the extremity of the vertebral axis. 



6. In the recent Ceratodus Fursteri Krefit, the tail is diphycercal (secondary 

 dipliycercy), the median fins are continuous, the pectorals and ventrals retain the 

 biserial archipterygium, but the cranial roof-bones have become few. 



7. In Pvotojiterus annecte-ns Owen, the body is more eel-like, and the paired 

 fins have lost the lanceolate leatlike appearance which they show in Ceratodiis 

 and the older Dipnoi. They are like slender filaments in shape, with a fringe on 

 one side of minute dermal rays ; internally they retain the central jointed axis of 

 the '■ archipterygium,' but according to Wiedersheim the radials are gone, except 

 it may be one pair at the very base of the filament. 



8. Finally in Lepidusiren jMradoxa Fitz. the paired fins are still more reduced, 

 having become very small and short, with only the axis remaining. 



From this point of view, then, Dipterus, instead of being the most specialised 

 Dipnoan, is the most archaic, and the modern Ceratodiis, Protojjtencs, and Lept- 

 dosiren are degenerate forms, and instead of the Crossopterygii being the offspring 

 of Dipferus-like forms, it is exactly the other way, the Dipnoi owing their origin 

 to Holoptychiidfe, which again are a specialisation on the Rhizodontidae, though 

 they did not survive so long as these in geological time. Consequently the 

 Ceratodiis limb, with its long median segmented axis and biserial arrangement of 

 radials, is not an archypterygium in the literal sense of the word, but a deri- 

 vative form traceable to the short uniserial type in the Rhizodonts, But from 

 ■what form of fin that was derived is a question to which palajontology gives 

 us no answer, for the progenitors of the Crossopterygii are as yet unknown to us. 



Plausible and attractive as this theory undoubtedly is, and though it relieves 

 the palaeontologist from many difficulties which force themselves upon his mind 

 if he tries to abide by the belief that the Dipnoan form of limb had a selachian 

 origin, and was in turn handed on by them to the Crossopterygii, yet it is not 

 without its own stumbling-blocks. 



First as to the dentition, on which, however, M. Dollo does not seem to put 

 much stress, it is impossible to derive Dipterus directhj from the Holoptychiidse, 

 unless it suddenly acquired, as so many of us have to do as we grow older, a 

 new set of teeth. The dendrodont dentition of Holoptychhis could not in any 

 way be transformed into the ctenodont or ceratodont one of Dipterus : both are 

 highly specialised conditions, but in different directions. Semon has recently 

 shown that the tooth-plates of the recent Ceratodus arise from the concrescence of 

 numerous small simple conical teeth, at first separate from each other.' Now 

 this stage in the embryo of the recent form represents to some extent the con- 

 dition in the Uronemidse of the Carboniferous and Lower Permian, which stand 

 quite in the middle of Dollo's series. 



Again, the idea of the origin of the Dipnoi from the Crossopterygii in the 

 manner sketched above cuts ofi' every thought of a genetic connection between 

 the biserial archipterygium in them and in the Pleuracantbidse, so that we should 

 have to believe that this very peculiar type of limb arose independently in the 

 Selachii as a parallel development. It may be asked, "VVhy not ? We may feel 

 perfectly assured that the autostylic condition of the skull in the Holocephali 



' 'Die Za-h-nentY/ickelwrig des Ceratodv 8 Forste7-i.' Jena, 1899. 



