IXTEODUCTIOX. xiii 



condition, but so far as the fore paddle is concerned it is believed that the specimen 



figured on PI. II. fig. 6 represents as nearly as possible the actual arrangement of the 



bones, every piece having been carefully numbered and a sketch-plan of their 



arrangement having been made before their removal from the matrix. The circumstance 



that the ossicles do not fit together in a close pavement, as in the paddles of 



Ichthyosaurus, but were surrounded by a considerable amount of cartilage, adds to the 



uncertainty as to the precise arrangement. It can be seen that the intermedium 



supported two digits, as in the typical latipinnate forms (e. g., Ichthyosaurus communis, 



I. intermeclius) ; but the paddle differs from those of the earlier forms owing to the 



fact that its width has been still further increased by the great enlargement of the 



pisiform, which has acquired an articulation with the distal end of the humerus, in 



some cases almost as large as that possessed by the radius; the row of ossicles 



supported by the pisiform become enlarged and form a well-developed digit ; a 



preaxial row of small sesamoid ossicles may also be present. There are traces of the 



widening of the paddle by the increased size of the pisiform and its digit even in 



the Triassic Mixosaurus, and probably some early Jurassic descendant of that genus 



is the ancestor of OiMhalmosaurus, though at present no species is known to which 



that position can be definitely assigned. There seems to be no reason for the 



suggestion that Ophthalmosaurus is descended from Shastasaurus, for in that Triassic 



genus the digits have already undergone much reduction, there being probably, 



according to Merriam, only two large digits and one reduced digit in the manus. In 



the later forms of Ichthyosaurus the broadening of the fore paddle which occurs in 



Ophthalmosaurus may be effected in other ways ; thus in Ichthyosaurus extremus, 



described by Bonlenger * from an unknown locality and horizon, but now known to be 



almost certainly of Kirameridgian age, the intermedium is thrust between the radius 



and ulna, and articulates with the humerus by a well-marked facet, so that that bone 



comes to resemble closely the humerus of Ophthalmosaurus, and if found isolated 



might be mistaken for it. The width of the paddle in this case is also added to by 



the presence of a row of sesamoid ossicles on both the preaxial and postaxial borders. 



Another method of widening is found in Ichthyosaurus platydactylus, described by 



Broilif from the Cretaceous (Aptian) of Hanover; in this species, although it is 



* Proc. Zool. Soc. vol. i. (1904) p. 424, 



t 'Falieontograpbica,' vol. 54 (1907-8) p. 139, pis. xii. & .\iii. 



