THE PLESIOSAURIA. 185 



vicular, elements in any P/mos^z«rw5, though they appear to 

 have been well developed in JSfothosaurus. 



The humerus is a stout bone — prismatic, and with a rounded 

 head at its proximal end, flattened and broad distally. (Fig. 

 68, B.) Its anterior margin is nearly straight, or even slightly 

 convex, while the posterior is concave. Distally, it presents 

 two facets, meeting at an angle, with which tiie broad and 

 short radius and ulna articulate. The ulna differs in shape 

 from the radius, being convex posteriorly, and concave in 

 front. The two bones are of equal length, and much shorter 

 than the humerus. There are six rounded carpal bones,* 

 arranged in two rows ; and to these succeed five digits, com- 

 posed of metacarpals and phalanges, which are elongated and 

 constricted in the middle. The middle digits have numerous 

 phalanges. 



The pelvic arch has very large dimensions, in correspond- 

 ence with the size of the hind-limb, which is usually longer 

 than the fore-limb. (Fig. 68, F and D.) The ilium is a 

 vertically elongated bone, narrower below than above, where 

 it becomes connected with the sacral ribs. Inferiorly, it unites 

 with the pubis and with the ischium, to form the acetabulum. 

 The pubes are very broad quadrate bones, much larger than 

 the ischia, and they meet in a median symphysis. The ischia, 

 triangular and expanded, also unite in a ventral symphysis. 

 The femur resembles the humerus in its general form, although 

 both its sides are straighter, and the other bones of the hind- 

 limb are so like those of the fore-limb, as to need no special 

 description. 



There can be little doubt that all the bones of the limbs 

 were, like those of the Cetacea^ enclosed within a common 

 sheath of integument, so as to form a paddle. 



Such is the general organization of the skeleton of the 

 Plesiosauria, which are long extinct animals, entirely confined 

 to the Mesozoic Rocks, from the Trias to the Chalk, inclusive. 

 They may be divided into two groujDS, according as they are 

 Triassic, or Post-Triassic, in age. 



The Post-Triassic group contains the genera Plesiosaicrus 

 and Pliosaurus^ the different species of which appear to differ 

 in little more than the proportions of the head to the trunk, 

 and the relative length and degree of excavation of the centra 



* It may be a question wlietlier the fourth distal bone in the carpus and 

 tarsus (Fig. 68, B and D) belongs to carpus and tarsus, or to metacarpus and 

 metatarsus ; or whether it is formed by the confluence of elements belonging 

 to both regions. 



