80 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



number of the phalanges and the form of the paddle have generally varied, 

 so that the many fingered paddle of /. com mini is is broad but short, and the 

 paddles of I. acutirostris are excessively long and slender. The two types of 

 paddles represented by I. communis on the one hand, and I. platyodon with 

 /. iindirostris on the other hand, distinguish two groups of ichthyosaurs which 

 are in the main fairly distinct (figs. 103 and 104). They were separated by 

 Kiprijanoff 37 as Latipiunipedes typified by I. communis, and Longipinnipedes 

 represented by /. platyodon; but were not designated as genera, as there seemed 

 to be no point at which a dividing line could be drawn satisfactorily. This 

 classification was accepted in somewhat modified form, by Lydekker 38 in pre- 

 paring his catalogue of fossil reptiles of the British Museum, the group names 

 being changed to Latipinnati and Longipinnati. The groups have been recog- 

 nized by Fraas and other investigators who have made particularly close 

 studies of the structure and relationships of the Ichthyosauria. Fraas accepted 

 the divisions Longipinnati and Latipinnati but treated them as groups, and not 

 genera, as had been done by Kiprijanoff and Lydekker. More recently they 

 have been designated as distinct genera by Jackel 39 and Boulenger. 40 



If the longipinnate and latipinnate divisions of the ichthyosaurs of the 

 Lower Lias actually represent distinct groups of species tending to specialize in 

 somewhat different directions, they should be recognized as separate genera. 

 It is, however, true that most of the investigators who have worked over large 

 collections have found the separation of the two divisions so difficult that they 

 have refrained from giving generic values to them. 



To a considerable extent the same difficulties that are met in this grouping of 

 Jurassic species of the true ichthyosaurs are found in the definition of other 

 genera and species. The species of Opthcdtnosaurus and Baptanodon appear 

 quite widely separated from the typical forms, but in reality most of the char- 

 acters which separate them from Ichthyosaurus are to be found in various 

 forms of Ichthyosaurus itself. 



The statements which have been made regarding variation in the structure 

 of the paddles might apply almost as well to most other portions of the skeleton 

 in which characters of generic or specific rank appear, as in the facial region 

 of the skull, the dentition, and the caudal fin. A general study of the consid- 

 erable variations occurring inside the Ichthyosauria seems to show fewer inter- 

 ruptions between the various forms recognized than commonly appear in 

 natural groups. These facts apparently indicate a high degree of plasticity 



'? Kiprijanoff, M., Mem. 1 'acad. imp. d. sc. St. Petersburg, VII ser., t. 28, no. 8, p. 88. 



ss Lydekker, E., Geol. Mag. London, 1888, Dec. 3, vol. 5, p. 310. Also Catlg. Foss. Eept. and Amphib. Brit. 

 Mus., pt. 2, p. 13, 1889. 



3 Jackel, O., Marz-Protokoll d. Deutsch, geol. Gesel., Bd. 56, Jahrgang, 1904, p. 32. Jackel proposed the 

 name Eurypterygius for the latipinnates, and Stenopterygitis for the longipinnates. 



"> Boulenger, G. A., Proc. Zool. Soc. Lond., vol. 1, p. 425. Aug. 2, 1904. Boulenger uses the generic name 

 Ichthyosaurus, sensu stricto, for the Latipinnati; and Proteosaunis Home for the Longipinnati. 



