228 PALEONTOLOGY 



with a convex head, sub-cylindrical at its proximal end, 



becoming flattened and gradually expanded 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 appear- 

 ing to have contained more unossified matter. The meta- 

 carpals, five in number, are elongate, slender, slightly expanded 

 at the two ends, flattened, and sometimes 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 com- 

 mon sheath of integument. The pelvic arch consists of a 

 short but strong and straight narrow moveable ilium, and of a 

 broad and flat pubis and ischium ; the former subquadrate or 

 subcircular, the latter triangular ; the fore-and-aft expanse <>l 

 both bones nearly equals that of the coracoids. All concur in 

 the formation of the hip-joint, The ischium and pubis again 

 unite together near their mesial borders, leaving a wide elliptic 

 vacuity, or " foramen ovale," between this junction and their 

 outer acetabular one. The pelvic paddle is usually of equal 

 length with the pectoral one, but in P. macrocephedus it is 

 longer. The bones closely correspond, in number, arrange- 



