MUE^NOSAUEUS. 113 



covered with cartilage in life. The shaft is somewhat compressed from above 

 downwards, its narrowest part being about the middle of its length, where it is oval in 

 section. On the ventral face near the preaxial border and a little above the middle of 

 the shaft is a strongly marked rugose surface for the attachment of muscles ; iu old 

 individuals much of the upper part of the ventral face of the bone, both above and 

 below this surface, is considerably roughened by ridges and tubercles. On both the 

 ulnar and radial borders of the shaft there are smaller rugosities for muscle-attachment. 

 Both the anterior and posterior borders of the bone are concave, the posterior concavity 

 being the most marked. The distal end is compressed dorso-ventrally and much 

 expanded, especially in old individuals in which ossification is far advanced, but, as 

 mentioned above, the expansion is never so great as in the humerus of Cryptocleidus. 

 At its distal end the humerus articulates with the radius and ulna, the surfaces for 

 which meet one another at a very obtuse angle. These surfaces are rough and in life 

 were no doubt covered with cartilage, which also extended along the postaxial and to 

 some degree the preaxial borders of the expansion ; it is possible that small ossifications 

 may have occurred in the pro-radial or more often in the post-ulnar cartilage. Such 

 an ossification is shown in the figure of the fore limb oi Tricleidus (text-fig. 77). The 

 surface of the humerus has a peculiar fibrous appearance, the direction of the "fibres" 

 being longitudinal. The middle of the shaft is nearly smooth, but towards the 

 extremities, especially on the distal expansion, the surface is pitted by a number of 

 small foramina running inwards towards the middle of the shaft. 



The general form of the radius (r., PI. IV. fig. 7 ; text-fig. 63, B) will be 

 best understood from the figures. The oblique proximal surface for union with the 

 humerus is gently convex, the comparatively thin anterior (preaxial) border is gently 

 concave, while the thickened uhiar (postaxial) border is sharply notched. Distally the 

 bone articulates with the intermedium by a short facet looking downwards and back- 

 wards, and with the radiale by a gently concave or flat surface. The ulna (u., PL IV. 

 fig. 7 ; text-fig. 63, B) is much smaller than the radius, particularly in length ; its 

 anterior (radial) border is deeply notched, and the notch with the corresponding one on 

 the radius encloses a foramen, representing the opening between the shafts of tlie two 

 bones before they had become so much shortened. The posterior border is convex, 

 while the distal bears two facets making a very obtuse angle with one another ; the 

 anterior facet, which is the larger, articulates with the intermedium, the posterior 

 with the ulnare. 



The proximal row of carjjals (text-fig. 63, B) consists in most cases of three elements 

 only, but sometimes there is a fourth, which is a small bone articulating with the 

 postero-external angle of the ulna and the postaxial portion of the proximal face of 

 the ulnare. This element may be a pisiform (p., text-fig. 63, B), and is probably 

 homologous with the third element which in some cases articulates with the humerus, 

 e. g., in Iricleidus and iJolichorhynchops and Polycotijlus. "Williston regards this ossicle 



Q 



