ISfoticos of Memoirs — Dr. R. H. Traquair's Address. 523 



conical teeth characteristic of the family are, on the palatal and 

 splenial bones, replaced by dental plates, i-eminding us of those 

 of the Dipnoi. Certainly the Platysomidce seem to me to form 

 a morphological series telling as strongly in favour of Descent as 

 any other in the domain of palseontology.^ 



If we now return to the Pala^oniscidce we find that they dwindled 

 away in numbers in the Jurassic rocks, and finally became extinct 

 at the close of that epoch. But already in the Lias (leaving the 

 Triassic Catopteridse out of consideration for the present) we find 

 that they have sent off another offshoot sufficiently distinct to be 

 reckoned as a new and separate family, namely, the Chondrosteid^, 

 in which the path of degeneration, in all but the matter of size, 

 seems to have been entered on. 



In the genus Chondrosteiis, though the palteoniscid type is clearly 

 traceable in the cranial structure, there is marked degeneration as 

 regards the auiount of ossification, and though the suspensorium 

 is still obliquely directed backward the toothless jaws are com- 

 paratively short, and the mouth seems now to have become tucked 

 in under the snout as in the recent sturgeon. Then the scales have 

 entirely disappeared from the skin except on the upper lobe of tlie 

 heterocercal caudal fin, where they are still found arranged exactly 

 as in the Paleeoniscidce. 



Chondrosteiis in fact conducts us to the recent Acipenseroids — the 

 Polyodontida3 (Paddle-fishes) and Acipenseridee (Sturgeons). So 

 the sturgeons and paddle-fishes of the present day would seem to be 

 the degenerate, though bulky, descendants of the once extensively 

 developed group of Palceoniscidte, even as the modern Dipnoi are 

 degenerated from those of Pala30zoic times. 



In the Upper Permian occurs the genus Acentrophorus, whose 

 fellowship with Semionotus, Lepidotiis, and all the rest of the series 

 of Mesozoic semi-heterocercal ' Ganoids ' is at once obvious. If 

 we look at the configuration of a typical Jurassic member of this 

 series, such as Lepidolas or Eiignathus, we sliall at once see that we 

 are a stage nearer the modern osseous fish. Though the scales are 

 bony, rhombic, and ganoid, we are struck by the ' Teleostean '-like 

 aspect of the external bones and plates of the head, the rays of the 

 dorsal and anal fins are fewer and correspond in their number to that 

 of the internal supports or ' iuterspiuous ' bones, while in the caudal 

 we see the semi-heterocercal or abbreviate-heterocercal condition. 



Then, if we refer to the tail of Lepidosteus itself, we shall observe 

 how few are its rays and how evident it is that we have here to do 

 only with the lower lobe of the original paUconiscoid caudal fin. 

 For a convincing corroboration of this we have only to look at the 

 tail of the embryo Lepidosteus as described and figured by Professor 

 A. Agassiz to see that it in reality passed through an Acipenseroid 

 stage, and the last we see of the upper lobe of this tail is in the 

 form of a filament which projects from the top of the original lower 

 lobe and then disappears. 



1 R. II. Traqnair, " Structure and Affinities of the Platysomi(l;u " : Trans. Eoy. 

 Soc. Edin., vol. xxix (1879), pp. 343-391. 



