10 FOSSIL REPTILIA OF THE 
of from side to side, increasing in breadth and diminishing in thickness to the distal 
articular end. The surfaces for tibia (66) and fibula (67) are indicated by, or meet at, 
a widely open angle. The projecting part of the femur beyond the tibial surface is 
rounded off; that beyond the fibula is, as above remarked, of greater extent, and may 
have terminated more angularly, but the extreme end has been broken away (fig. 2, 65). 
The representatives of tibia and fibula appear in size and shape, as in Jchthyosaurus, to 
be a first series of tarsal ossicles ; they, however, markedly exceed in size the ossicles of 
the two succeeding rows, properly constituting the tarsal segment of the fin. The bone 
(66) answering to the tibia in Plesiosaurus (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 than the tibia, measuring 1 inch 9 lines in length, and 
2 inches 8 lines in breadth: 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 a and el’, and this slight modification is interesting 
because the homologous bone in Plestosaurus (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 arrangement 
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 
Plesiosauri (Pl. 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 Pliosaurus 
portlandicus is relatively larger and less triangular in shape than in Plestosaurus 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 Plescosauri, the innermost or scaphoid (s) is, however, the smallest: it is trans- 
versely elliptical in shape, 1 inch 9 lines in breath, 1 inch 2 lines in length ; the original 
ligamentous interspace between it and the tibia is 3 lines. ‘The astragalus (4) has a pro- 
duced 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 (c/’) is the largest of this row; its proximal margin is straight and 
