54 MAEINE EEPTILES OF THE OXEOED CLAY. 



described by Mr. Boulenger* as the type of Iclitlujoscmrus extremus it is clear that 

 the third bone is the intermedium which is thrust between the radius and ulna, 

 and articulates extensively with the humerus ; this same condition is known in other 

 species of Ichthyosaurus and appears in some cases to occur also in the hind paddle f. 

 Beneath the humerus the remaining bones of the paddle form a mosaic (text-fig. 37), 

 the proximal rows having been separated by comparatively thin layers of cartilage, 

 while th(! more distal phalanges seem to have been embedded in a mass of that 

 substance and separated from one another by considerable intervals. The radius (text- 

 tig. 37, r.) is a pentangular or subquadrate bone, the anterior (outer) border of which 

 is thin and sharp and continues the line of the radial border of the humerus ; 

 internally the bone thickens greatly. Proximally it unites with the humerus and 

 internally (postaxially) it usually touches the ulna, but the length of its contact with 

 that bone varies greatly in different individuals according to the degree to whicli the 

 intermedium is thrust between them. Postero-internally and distally it has long 

 siufaces for the intermedium {int.) and the radiale (?-fff?.) respectively. The ulna (id.) is 

 a large subquadrate bone articulating with the humerus, usually to a greater or less 

 degree with the radius, as already mentioned, with the intermedium and ulnare, and, 

 finally, posteriorly with the j^^siform {i).). This, the third element articidating with 

 the humerus, is of a somewhat doubtful character, though the interpretation here 

 followed is now usually adopted. Prof. Seeley, in his original paper on Ojphthahno- 

 saurus, called it the olecranon, and recently Prof. Williston has proposed for it the 

 non-committal name — epipodial supernumerary. This element is oval, and its posterior 

 border is thin, though it seems to have continued fringed with cartilage throughout 

 life : the extent of its articulation with the humerus, as above noted, varies much, but 

 in all cases is comparatively small. It may be stated at once that distally the pisiform 

 articulates with a smaller oval element, which in its turn bears a small distal nodular 

 bone (in some cases two), the Avhole apparently representing a rudimentary fifth digit. 

 Leaving the pisiform and its appendages out of account, the manus may be described 

 as consisting of a carpus (the proximal row of which consists of three bones, the distal 

 of four), and four digits composed of a varying number of bones. A certain degree of 

 variation in the arrangement of the carpal bones occurs, but that here described seems 

 to be the normal one. Unfortunately in the paddle figured the second carpal of the 

 distal row is much smaller than usual and presents somewhat the appearance of a 

 centrale articulating between the intermedium and radiale, but its true character is 



* Proc. Zool. Soe. ]904, vol. i. p. -iHi. The horizon and locality from which Mr. Boulenger's specimen 

 came are not kuown, and he regarded the species as probably Liassic ; but a humerus of precisely similar 

 type was lately obtained from the Kinimericlge Clay of Swindon by Mr. Arthur D. Passmore, so that it is 

 likely that the type specimen is also from the later horizon, as from its extreme specialization would be 

 probable. 



t See Lydekl;er, Catal. I'oss. Eept. Brit. Mus. pt. ii. p. 43, test-fig. 20. Also Fraas, 'Die Ichthyosaurier 

 d. Suddeutschen Trias- und Jiir.i-Ablagerungen,' p], vi. fig. 4. 



