OCR Text |
Show 1904.] SPECIES OF ICHTHYOSAUR FROM BATH. 425 (pisiform), 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 /. quadriscissus. Besides, the compact arrangement of the phalanges shows a nearer relation to Ichthyoscmrus than to Ophthalmosaurus. 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 Mixosaurus, through the latipinnate Ichthyosaurus (e. g. /. communis) 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 Ichthyosaurus = Proteosaumts Home), represent a distinct series, likewise extending from 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 Baptanodontidae. J C E A S S I C TDorsal ribs wideband •{ forked ; pelvis and C E E T A C E O U S . (__ ischium narrow. fDorsal ribs widely | forked ; pelvis and ischium plate-like. TRIASSIC. < Dorsal ribs single-headed or feebly forked; pelvis and L ischium plate-like. Proteosaurns. Ichthyosaurus. Ophthalmosaurus. Toretocnemus. Merriamia f. Mixosaurus. Shastasaurus? Hypothetical forms connecting the Rhynchocephalia with the Ichthyosauria. * [The complete account of the new species described in this communication appears here; but as the name and preliminary diagnosis were published in the 'Abstract,' the new species is distinguished by being underlined.-EDITOE.J f The name Leptochirus J. C. Merriain, 1903, being preoccupied, that of Merriamia is here proposed to replace it. |