SPECIES OF ICHTHYOSAUR FROM BATH. 



425 



1904.] 



(pisifoi-m), whilst in the present case the additional facet received^ 

 the intermedium, which penetrates between the much reduced" 

 epipodials, as is known to sometimes happen in the hind limb of 

 Ichthyosaurus communis and /. quaclriscissus. Besides, the 

 compact arrangement of the phalanges shows a nearer relation to 

 Ichthyosaurus than to Ophthahnosaui'tis. In fact this paddle 

 appears to represent the extreme of the latipinnate group for 

 which the generic name Ichthyosaurus, sensu stricto, is to be 

 retained, and is characterised by the development of both radial 

 and ulnar sesamoid bones. 



Mr. Boulenger pointed out that an evolutionary series could 

 be traced between the most generalised and geologically oldest of 

 Ichthyosaurs, the Triassic Ilixosaurus, through the latipinnate 

 Ichthyosaurus (e. g. /. commtmis) to the type here described and 

 for which the name Ichthyosaurus extremus * {cf. Abstr. P. Z. S. 

 1904, No. 5, p. 18, March 22) was proposed. Whilst the forms 

 in which a single row of carpals and of phalanges correspond 

 to the intermedium, instead of two {Leptochirus = Merriamia, 

 Toretocnemus, longipinnate Ichthyosazirus = Proteosaurus Home), 

 represent a distinct series, likewise extending fi-om the Upper 

 Trias (California) to the Upper Jurassic (Solenhofen). 



Ophthalmosaurus may be regarded as probably the terminal 

 form of a third series, as shown in the following diagram. The 

 paddles of Shastasaurus, from the Upper Trias of California, are 

 yet imperfectly known, but the genus is provisionally placed by 

 O. P. Hay in the Baptanodontidse. 



JuBASSic fDorsal ribs widel.v 

 and ■{ forked ; pelvis and 



Ceetaceotjs. l^ ischium narrow. 



Teiassic. < 



Dorsal ribs widelj' 

 forked ; pelvis and 

 ischium plate-like. 



Dorsal ribs single- 

 headed or feeblj' 

 forked ; pelvis and 

 L ischium plate-like. 



Proteosaurus. Ichthyosaurus. Ophthahnosaurtis. 



Toretocnemus. 



Ilerriamia f. Mixosaurus. Shastasaurus ? 



Hj'pothetical forms connecting the 

 Rhj'nchocephalia with the Ichthyosauria. 



* [The complete account of the new species described in this communication 

 appears here ; but as the name and preliminarj- diagnosis were published in the 

 ' Abstract,' the new species is distinguished by being underlined.— Editor.] 



t The name Leptochirus J. C. Merriam, 1903, being preoccupied, that ot 

 Merriaynia is here proposed to replace it. 



