Notices of Memoirs — Dr. R. II. Traquairs Address. 517 



In my description of the pectoral fin of the Carboniferous Cladodus 

 Neilsoni,^ I have shown that the cartilaginous structures apparently 

 present an uniserial archipterygium intermediate between the arrange- 

 ment in Phur acanthus 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 transition 

 from the ' ptychopterygium ' of Cladoselache to the Pleuracanth and 

 Dipnoan limb. And so, in fact, this fin of Cladodus is claimed in 

 support of their views by both parties in the dispute. 



When we add that Semon emphaticall}'^ 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 Ilybodus,^ supports Gegenbaur's theory, it will be 

 seen that Elasmobranch palaeontology 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 characterized 

 externally by their jugular plates and lobate paired fins, and repre- 

 sented in the present day only by the African genera Polypterus and 

 Calamoichthys, which together form the peculiar family Polypteridae. 

 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 Palteozoic times — the Osteo- 

 lepidae, Khizodontidas, Holoptychiida?, and Coelacanthidte — but it 

 is only with the first three that we have at present to deal. The 

 Osteolepidas and Pihizodontidfe, which appear together in Middle 

 and die out together in Upper Palteozoic times, resemble each other 

 very closely. In both we have the paired fins, more especially the 

 pectoral, obtusely or subacutely lobate ; there are two separate dorsal 

 tins, one anal, and the caudal, which is usually heterocercal, though 

 in some genera it is more or less diphycercal. In both the teeth are 

 conical and have the same complex structure, the dentine being 

 towards the base thrown into vertical labyrinthic folds, exactly as 

 in the Stegocephalian Labyrinthodonts, and this along with the 

 lung-like development of the double air-bladder in the recent 

 Polypteridas has given rise to the view that from these forms the 

 Stegocephalia have originated. The nasal openings must have been 

 on the under surface of the snout, as in the Dipnoi. 



Of these two so closely allied families we must conclude that the 

 Osteolepidas are the more primitive, as in them the scales are acutely 

 rhombic and usually covered with a thick layer of ganoine, while 



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



- " Die Entwickelung der paarigen Flosseii des Ceratodiis Furstcri " ; Jena, 1898. 



* " Ueber das Genus Kyhodm iind seine systematisciie Stellung" : PaUuonto- 

 graphica, vol. xlvi (1900). 



* Five, if wo include the singular and still imperfectly known Tarrasiid;c of the 

 Lower Carboniferous. 



