248 PALEONTOLOGY. 



median process is short ; a longitudinal ridge projects from 

 the middle of the internal surface. The humerus is a moder- 

 ately thick and long bone, with a convex head, sub-cylindrical 

 at its proximal end, becoming flattened and gradually ex- 

 panded to its distal end, where it is divided into two indistinct 

 surfaces for the radius and ulna. The shaft in most species 

 is slightly curved backwards, or the hind border is concave, 

 whilst the front one is straight. The radius and ulna are 

 about half the length of the humerus ; the former is straight, 

 the latter curved or reniform, with the concavity towards the 

 radius ; both are flattened ; the radius is a little contracted 

 towards its carpal end, and in some species is longer than the 

 ulna. The carpus consists of a double row of flat rounded 

 discs, — the largest at the radial side of the wrist ; the ulnar 

 or hinder side appearing to have contained more unossified 

 matter. The metacarpals, five in number, are elongate, slen- 

 der, slightly expanded at the two ends, flattened, and some- 

 times a little bent. The phalanges of the five digits have a 

 similar form, but are smaller, and progressively decrease in 

 size ; the expansion of the two ends, which are truncate, makes 

 the sides or margins concave. The first or radial digit has 

 generally three phalanges, the second from five to seven, the 

 third eight or nine, the fourth eight, the fifth five or six 

 phalanges. All are flattened ; the terminal ones are nailless ; 

 and the whole were obviously included, like the paddle of 

 the porpoise and turtle, in a common sheath of integument. 



The pelvic arch consists of a short but strong and straight 

 ilium, of a broad and flat subquadrate pubis and of a triangular 

 ischium, the fore-and-aft expanse of the last two bones nearly 

 equals that of the coracoids. All concur in the formation of 

 the hip-joint. The pelvic paddle is usually of equal length 

 with the pectoral one, but in P. macrocephalus it is longer. 

 The bones closely correspond, in number, arrangement, and 

 form, with those of the fore limb. The femur has the hind 



