MESOSAUEIA OF SOUTH AFEICA. 595 



The anterior limbs are well preserved, and extended transversely. 

 The measurement over them exceeds 15 centimetres, and each 

 limb is 6| centimetres long. 



The humerus is 2*625 centimetres long. It has the form of the 

 bone in Pliosaurus, being deep at the proximal end, elongated, 

 nearly straight on the anterior border, and concave on the posterior 

 border in consequence of the distal expansion. In no way except 

 as a genus does it differ in form from the humerus of the Edentate 

 mammal Megcdonyx, where that bone has lost its terminal epiphyses, 

 thus showing a new example of evolution in ossification which is 

 associated with transition from one vertebrate type to another. The 

 bone is | centimetre wide at the proximal end, and nearly 1 centimetre 

 wide at the distal end. The anterior border is flattened proximally, 

 but becomes compressed distally to a sharp edge ; the posterior border 

 is modified in the same way. The under side of the bone is concave 

 in length. The distal end is truncated, slightly convex from front 

 to back. Its inferior margin is slightly thickened, and its articular 

 surface shows two concavities which correspoDd in position with 

 the heads of the ulna and radius, though those bones on both sides 

 appear to be separated from the hum-arus by an interval of about g 

 centimetre. The bone has a comparatively large ent-epicondylar 

 foramen which passes from the internal or posterior border obliquely 

 downward and forward so as to open on the under side of the bone 

 near to the posterior margin, above the ulnar articulation. It is 

 vertically ovate and has not the narrowj elongate form figured 

 by Gervais in the type of Mesosaurus tenuidens. This, with the 

 slenderness of the humerus, supports the indication of the cervical 

 ribs that the species may be distinct. 



The ulna and radius are exceptionally slender in proportion to 

 the size of the humerus. The radius is 1| centimetre long, straight, 

 slightly enlarged at both extremities, with the distal end com- 

 pressed so as to carry a median ridge on the under side, which may 

 make the distal end triangular. 



The ulna is a little shorter. It is very slightly curved, the proxi- 

 mal end being a little developed on its radial side, where the two 

 bones of the forearm are in contact. And distally there is a 

 corresponding development towards the carpal element which is 

 wedged between the radius and ulna so as to separate their distal 

 ends. The posterior outline of the ulna is very slightly convex, 

 and its radial border is concave. In slender form it is the counterpart 

 of the radius. The transverse measurement over the proximal ends 

 of the two bones as they lie together is | centimetre ; the transverse 

 measurement over the distal ends as they lie is | centimetre. 



The carpus is remarkable for the large size and triangular 

 arrangement of the three bones which form the proximal row, as 

 compared with the small size and linear arrangement of the four 

 bones in the distal row. 



The proximal carpal, which alone gives attachment to the radius 

 and ulna, may be regarded as the lunar bone. It is flattened, of 

 irregular subquadrate outline, and has the border towards the 



