PORTLANDIAN PLIOSAURS. 165 



extent, aud may have terminated more angularly, )jut the extreme end has 

 been broken away. The representatives of tibia and fibula appear in size and 

 shape, as in Ichthyosaurus, to be a first series of tarsal ossicles ; they, however, 

 markedly exceed in size the ossicles of the two succeeding rows, properly consti- 

 tuting the tarsal segment of the fin. The bone (66) answering to the tibia in 

 Plesiosaurus (PI. 23, fig. 4, (66)) is an irregular oval or oblong flat plate, the margin 

 adapted to the femur being longest and least convex. The breadth of this bone 

 exceeds its length, and the inner or tibial, and the outer or fibular, margins are 

 rounded or strongly convex ; the distal margin is more even or straight at its 

 middle part. The length of the bone (in the axis of the femur) is 2 inches ; the 

 breadth of the bone is 2 inches 9 lines : an interval of 5 lines between it and the 

 femur indicates most probably the thickness of ligamentous matter which dissolved^ 

 away after the carcase of the Reptile had sunk into the fine sand or sandy mud 

 now hardened into Portland stone. 



The fibula (67) is less, transversely, than the tibia : the margin towards the femur 

 is almost straight ; the outer and inner margins convex ; the distal one is produced 

 into a low rounded angle opposite the interspace between the tarsal bones « and 

 cl', and this slight modification is interesting because the homologous bone in 

 Plesiosaurus (fig. 4, 67) shows a similar angular production between the same 

 tarsal ossicles, whilst the distal end of the tibia is truncate. 



Another character which would seem to show that a tarsal structure or arrange- 

 ment immediately followed the femur is evidenced by a depression in the matrix 

 indicative of a third bone, smaller than either fibula or tibia, and of an oval form 

 with the long axis parallel with that of the fin and the small end of the oval 

 produced towards the femur. This ossicle I regard as the homologue of the 

 fabella (67'), which is present in some Plesiosaurl (PI. rugosus, for example, fig. 

 4, 67'), where its homotype in the fore-limb is represented by a detached olecranal 

 process of the ulna. But the bone (67') in Pllosaurus 2^ortlandicus is relatively 

 larger and less triangular in shape than in Plesiosaurus rugosus. 



The thickness of these tarsal-like representatives of tibia and fibula is about 

 4 lines. 



The three bones of the proximal tarsal row are more uniform in size and shape 

 than in most Plesiosauri, the innermost or scaphoid («) is, however, the smallest : 

 it is transversely elliptical in shape, 1 inch 9 lines in breadth, 1 inch 2 lines in 

 length ; the original ligamentous interspace between it and the tibia is 3 lines. 

 The astragalus («) has a produced part of its proximal margin directed toward the 

 interspace between the tibia and fibula : this modification somewhat interferes with 

 the regularity of its elliptical contour. Its length is 1 inch 4 lines ; its breadth 1 

 inch 11 lines. The interspace between it and the scaphoid is reduced to 2 lines ; 

 that between it and the cnemial bones is from 4 to 5 lines. The calcaneum {cl) is 

 the largest of this row ; its proximal margin is straight and parallel. 



