INTEODUCTION. 



XI 



species * probably of this age is known from New Zealand, showing that very early in 

 its history the group had become cosmopolitan. Triassie Ichthyosaurs from Spitz- 

 bergen were first described by Hulke f in 1873 under the names Ichthyosaurus 

 nordensldoldi and /. polaris ; these species have since been referred by Dames J to 

 Mixosaurus and by Yakolew § and Merriam || to the American genera Cymhospondylus 

 and Shastasaurus. In a paper lately published Wiman ^ describes a quantity of new 

 material, and finds that while I. nordensldoldi is referable to Mixosaurus, I. polaris is 

 to be placed in a distinct genus, Pessosaurus. At the same time he describes a new 

 genus, Pessopteryx, including several species. In Wiman's paper (p. 131, fig. 3) there 

 is an interesting restoration of Mixosaurus, showing the form of the tail-fin, which in 

 many of these early forms was supported by elongated neural spines, and in some cases 

 chevrons, while the sharp deflexion of the posterior part of the tail found in later 

 forms was only slightly indicated. 



The most important series of Triassie Ichthyosaurs is from the United States, where 

 remains referred to several genera have been described in detail by Merriam **. These 

 are from the Middle Trias of Nevada and the Upper Trias of California. The principal 

 genera are Cymhospondylus, Toretocneinus, Merriamia f f , Delphinosatcrus, Shastasauncs, 

 several species of some of these genera being known. Merriam has elaborately 

 tabulated the characters distinguishing these Triassie Ichthyosaurs from the recent 

 types. Some of the more important differences between the Triassie forms and 

 Ophthalmosaurus are shown in the following table : — 



* J. Hector, " On the Fossil Eeptilia of JN'ew Zealand," Trans. New Zealand Inst. vol. 6 (1S74) p. 355. 



t J. "W. Hulke, " Memoraudum on some Fossil Vertebrate Eemains collected by the Swedish Expeditions 

 to Spitzbergen in 1864 and 1868," Bihang k. Svensk. Vet.-Akad. Handl. vol. i. (1873) no. 9. 



t W. Dames, " Die Ichthyopterygier der Triasformation," Sitzb. Akad. Wiss. Berlin, 1895, p. 1045. 



§ N. Takolew, " Neue Punde von Trias-Saurieru auf Spitzbergen," Verb. Euss.-Kais. Min. Gesell. 

 St. Petersb. ser. 2, vol. 40 (1902) p. 179. 



II J. C. Merriam, " Triassie Ichthyosauria," Mem. Univ. California, vol. i. no. 1 (1908). 



% C. Wiman, " Ichthyosaurier aus der Trias Spitzbergens," Bull. Geol. Inst. Upsala, vol. s. (1910) 

 p. 124. 



** J. C. Merriam, " Triassie Ichthyosauria, witli Special Eeference to the American Forms," Mem. Univ. 

 California, vol. i. no. 1 (1908). This memoir contains a very exhaustive account up to the date of its publi- 

 cation of all the known Triassie Ichthyosauria. The only important paper on the subject published since, 

 is that by Wiman referred to above. 



tt The name Merriamia was substituted for the preoccupied Leptoclieirus by Boulenger (Proe. Zool. Soc. 

 vol. i. (1904) p. 425). On page 3 of^the present volume Le^tocJieirus is employed, the correction not having 

 been made when this part was printed. 



52 



