AND CLASSIFICATION OP THE FOSSIL llEPTTLIA. 
2G'J 
The right tibia is a short, massive bone, which shows a small lateral surface at the 
proximal end for contact with the fibula. Its extreme length from the anterior 
jjroximal pi'ocess to the end of the internal distal talon is about 30 centirns. ; the 
least measurement between the proximal and distal articular surfaces at the posterior 
external angle is 15 or 16 centirns. 
The proximal surface is sub-triangular, with the posterior and external borders 
straight, each about 13 centirns. long, and meeting at a right angle ; with the 
external anterior border convex and rounding on to the adjacent sides. The trans¬ 
verse oblique measurement over this part of the proximal surface is nearly 20 centirns. 
The articular surface is flattened, but crossed from front to back by a blunt ridge, at 
about 7 or 8 centirns. from the outer border, which corresponds to the groove between 
the condyles of the femur, and makes a division of the pjroximal articular surface, which 
inclines the lateral surfaces to each other in a way seen in Mammals, with a slight 
anterior eminence extendino- between them. 
The smaller external condylar surface may have been supplemented by the fibula. 
There is some indication of the large convexity which extends anterior to the articular 
surfaces being formed by a separate ossification, in which case it might correspond to 
the patella, but the indication is so obscure that no weight can be attached to it. The 
anterior proximal border is rounded, and the posterior border is sharp and prominent. 
The shaft contracts so that its least measurements are below the middle, where the 
transverse width is 8 to 8'5 centirns. The sides preserve their individuality fairly 
well; and they expand distally to form the remarkable distal articulation, which has 
a transverse sub-ovate contour, but develops an anterior process on the middle of the 
anterior margin. The inner half of the surface is an oblong convexity which is pro¬ 
longed distally for 3 or 4 centirns. below the flattened external half of the articulation. 
The transverse measurement of the distal articulation is about 15 centirns., and the 
antero posterior measurement about 13 centirns. On the anterior border, a distinct 
notch in the middle reduces the antero-posterior measurement of the flattened part of 
the articulation to about 10 centirns. Thus, a large process, comparable to the talon 
of the Mammalian tibia is well defined, and, taken in connection with the character 
of the proximal end, gives a Mammalian character to the tibia which is unparalleled 
among Reptiles, and is nrore I'emarkable than the Mammalian character of the femur. 
There is no evidence of the genus to which this bone should be referred. It may 
be new or it may be Pareiasaiu'its or Tapinocej)haliis. 
Procolophon trigoniceps (Owen). (Plate 9, figs. 7, 8, 9.) 
No specimen has hitherto given an adequate idea of the structure of the skull in 
Procoloplion or evidence of its systematic position ; and, if I am able to Improve upon 
previous knowledge, it is because the skill of Mr. Richard Hall, Mason in the 
