LINXEAX SOCIETY OF LONDON. 37 



The greatest difficulties are met with in (liscoveriiif;; the 

 beginnings of any lineage and the fundaiuental points of diver- 

 genee. 1 have already alhided to some of these difficulties in 

 previous addresses when referring to certain fossil tishes, and I 

 wish now to mention three other striking instances which have 

 lately been studied. 



The Ichthyosauria, or fish-shaped reptiles, Avhich seem to have 

 lived in all seas thronghout the Mesozoic era or " age of reptiles," 

 appear suddenly in the Trias of both Europe and North America 

 ami Spitzbergen with their adaptations to strictly aquatic life 

 almost complete. These early forms only ditTer from later 

 Iciithyosauria in retaining more traces of their land ancestry, 

 such as the less deep overlapping of the bones in the skull, the 

 less uniform shape of the teelh, and the greater elongation of the 

 humerus, radius, and ulna in the fore limb. It must also be noted 

 that they are all of comparatively small size. (Some have the fore 

 limb or paddle rather elongated (longipinnate), others have it 

 rather broad (latipinnate), and in his recent monograph of the 

 Ichthyosauria * Baron von Huene points out that each of these 

 types begins a lineage \\ hich can be traced throughout the life 

 history of the group until its end at the close of the Cretaceous 

 period. The progressive changes in these two lineages are com- 

 paratively small. The bones of the slaiU soon become very deeply 

 overlapping, as in fishes and cetaceans, the teeth attain uniformity 

 and are implanted in a groove, and the vertical tail fin acquires 

 maximum effectiveness. It can only be observed that towards 

 the end of the Jurassic period the jaws sometimes become tooth- 

 less in the adult, the fore limbs are changed into more flexible 

 ])addles by the persistence of much cartilage round the bones in 

 the digits, and the lower lobe of the tail becomes more flexible by 

 the attenuation of the end of the vertebral axis which supports it. 

 Ichthyosaurs are, indeed, in all essentials the same from the 

 beginning to the end, and although Baron von Huene supposes 

 that the small Mesosaurians of the Permian period mav perhaps 

 represent their semi-aquatic ancestors, there is oiilj' slight basis 

 of fact for this hypothesis. AVith our present knowledge their 

 origin remains a mystery. 



The Pterosauria, or flying reptiles, have the same range in time 

 as the Ichthyosauria, and also exhibit remarkable uniformity. 

 Only a single skeleton has hitherto been found in the Trias, the 

 so-called Trihclesodon from northern Italy, so that the earliest 

 member of the grouj) is imperfectly known. The sj)ecimeii has 

 been studied lately by Baron F. Nopcsaf, who finds that in every 

 character which can be observed it is a well-formed Pterosauriaii. 

 Its fore limbs are already efficient wings, and the hind limbs only 



* F. von Hueue, " Die lehthvosiuirier des Lias und ilne Zus;uiiuieiili;ino-e " 

 Berlin, 1922. 



t F. Nopcsa, " Neubesclireibiing des Trias-Pteposaiiriers Tribelesoilon." 

 Pal^ont. Zeitschr. vol. v. (lt»22), pp. lGl-181, pi. ii. 



