X MAKIiN'E EEPTILES OP THE OXFOED CLAT. 



The Ichthj'osauis form a singularly homogeneous group, the earliest known forms 

 being already highly specialised for aquatic life, though some traces of a terrestrial 

 ancestr;y are retained. The first known member of the order is from the lower beds 

 of the Muschelkalk (Middle Trias) of Germany and Switzerland : this was first 

 described by Quenstedt* under the name IchtJii/osatirus atavus, and it has since been 

 discussed in detail by Fraasf and MerriamJ under the name Mixosaurus atavus. 

 Other remains of Ichthyopaurs from the Middle and Upper beds of the Muschelkalk 

 have been found in various European localities, but they are mostly fragmentary. From 

 the Upper Trias of Northern Italy (the Bituminous shales of Besano, in Lombardy) 

 excellently-preserved Ichthyosaurian skeletons are known and have been described 

 by Bassani § under the name I. cornaliamis. Baur || subsequently pointed out that this 

 species was more primitive than the later forms in several respects, and proposed to 

 place it in a separate genus Mixosaurus. The more important of the character's 

 that seem to point to a terrestrial ancestry are, first, the elongation of the epipodial 

 bones (radius and ulna, tibia and fibula) and their contraction in the middle to form 

 more or less of a shaft; and, second, the differentiation of the teeth, those in the 

 maxillary region being stout and blunt, those in the front of the jaws sharp and 

 conical. In the later forms the epipodials are shortened up and show' little or no trace 

 of ever having possessed a shaft, and the teeth are sharp, conical, and numerous, the 

 evolution of the Ichthyosaurs in this last respect, as in some others, showing an 

 interesting case of parallelism with that of the Toothed Whales. A more detailed 

 account of these Italian Ichthyosaurs has been given by llepossi ^ and by Merriam 

 {op. cit.). Remains of Triassic Ichthyosaurs are by no means confined to Europe: 

 numerous forms have been described from Spitzbergen and the United States, and one 



* F. A. Quenstedt, ' Petrefaliteukunde,' 1st ed. (1852) p. 129. 



t E. Fraas, 'Die Ichthyosaurier der Siiddeutschen Trias- uud Jura-Ablagerungen ' (1891) p. 37. 



+ J. C. Merriam, " Triassic Iclitbyosauria, with Special Eeference to the American Forms," Mem. 

 University of California, vol. i. no. 1 (1908) p. 90. 



§ F. Bassani, "Sul fossili e sull' eta degli Schisti bituminosi triasici di Besano," Atti Soe. Ital. Sci. Xat. 

 vol. 29 (1S86) p. 20. 



II G. Baur, " Ueber deu Ursprung der Estremitateu der Ichthyopterygia," Berichte ueber der XX. Yersam. 

 des Oberrh. geol. Ver. vol. xx. (1887). 



U F. Eepossi, " II Mixosauro degli strati triasici di Besano in Lombardia," Atti Soc. Ital. Sci. vol. 41 (1902) 

 p. 361. 



