TRANSACTIONS OF SECTION D. 775 



to tell. But their internal skeleton is from its nature not calculated for preserva- 

 tion, and for the most part we only Icuow those creatures from scattered teeth, 

 fin-spines, aud shagreen, specimens showiu<? either external configuiation or 

 internal structure beiDjr rare, especially in Palseozoic strata. But from what we do 

 know, there is no doubt that the ancient sharks were less specialised than those of 

 the present day. and that the recent Notidauids still preserve peculiarities which 

 were common in the Selachii of past ages. 



If we ask whether the fossil sharks throw any light on the disputed origin of 

 the paired limbs, whether from the specialisation of right and left lateral folds, or 

 whether that type of limb called ' archipterygium ' by Gegenbaur, consisting of 

 a central jointed axis with pre- and post-axial radial cartilage attached, was the 

 original form, I fear we get no very definite answer from Elasmobranch palteon- 

 tology. The paired lins of the Upper Devonian shark, Cladoselache, as described 

 by Bashford Dean, Smith Woodward, and others, seem to favour the lateral fold 

 theory, and Cope pointed to the right and left series of small intermediate spines 

 which in some Lower Devonian Acauthodei {Fare.nis and Climatius) extend 

 between the pectorals and veutrals as evidence of a former continuuas lat- ral 

 fin. So also, if 1 am right in looking on the lateral Haps of the Coelolepid;X) 

 as fins, the evidence ol the^e ancient Ostracodermi would be in the same 

 direction. 



But, on the other hand, we have the remarkable group of Pleuracanthidaj, 

 extending from the Lower Permian back to the Upper Devonian, in which the 

 paired tins are represented by an 'archipterygium ' which in the pectoral at least 

 is biserial. 



From this biserial 'archipterygium' in the Pleuracanthidse, Professor A. 

 Fritsch, ten years ago,' derived the tribasal arrangement of modern sharks, much 

 according to the Gegenbaurian method, effecting, however, a compromise with the 

 lateral fold theory by assuming that the Pleuracanth form originated from one, 

 consisting of simple parallel rods, like that described in Cladoselache. 



In my description of the pectoral fin of the Carboniferous Cladodus Neilsotii- 

 I have shown that the cartilaginous structures apparently present a uniserial 

 archipterygium intermediate between the arrangement in Fleuvacanthus and that in 

 the modern sharks, but I felt compelled to acknowledge that the specimen might 

 also be interpreted in exactly the opposite way, namely, as an example of a transi- 

 tion from the ' ptychopterygium ' of Cladosdache to the Pleuracanth and Dipnoan 

 limb. And so in fact this tin of Cladodus is claimed in support of their views by 

 both parties in the dispute. 



When we add that Semon emphatically denies that there is any proof for 

 considering that the pectoral fin of Cladoselache is primitive in its type,^ and that 

 Campbell Brown, in his recent paper on the Mesozoic genus Hi/boclus* supports 

 Gegenbaur's theory, it will be seen that Elasmobranch palfcontology has not as 

 yet uttered any very clear or decided voice on the question as to whether the so- 

 called archipterygium is the primary form of paired fin in the fish, or only a 

 secondary modification. We shall now inquire if we can obtain any more light 

 on the subject from the Crossopterygii and Dipnoi. 



The Crossopterygii are a group of Teleostomous fishes, characterised externally 

 by their jugular plates and lobate paired fins, and represented in the present day 

 only by the African genera Tolyptevus and Calamoichthys, which together form 

 the peculiar family Polypteridie. The Crossopterygii appear suddenly in the 

 middle of the Devonian period, their previous ancestry being unknown to us. 



Four families '•" are known to us in PalDsozoic times — the Osteolepidse, Rhizo- 



' 'Fauna der Gaskoble imd der Kalksteine der Permformation Bohmens,' vol. iii. 

 pt. i. (Prague. 1890), pp. 44-45. 



- Trans. Geol. Soc. (Glasgow), vol. xi. pt. i., 1897, pp. 41—50. 



' ' Die Entwickelung der paarigen Flossen des Ceratodus Forsteri.' Jena, 1898. 



* ' Ueber das Genus Hi/bodiis und seine systematische Stellung,' Palaonto- 

 graphica, vol. xlvi. 1900. 



'^ Fire, if we include the singular and still imperfectly known Tarrasiidje of the 

 Lower Carboniferous, 



