Structure of the Plesiosaurus Macrocephalus. 529 



assume the character of detached transverse processes as they approach the 

 tail. 



In general form and size, the ribs of PI. Macrocephalus resemble those of PL Hawkinsii ; 

 but they seem (in comparison with Hawkins's fig. 25) to be more suddenly expanded at their 

 upper extremities ; and the anterior ribs here present a bilobed appearance. 



The articular surface at the proximal or vertebral extremity of the longest ribs (which are about 

 nine in number) is oblong, convex, and roughened for the attachment of cartilage. 



Counting from the costal appendages, which change the hatchet-shape for the true rib-like form, 

 to the sacral vertebrae, there are twenty-two pairs of dorsal and lumbar ribs in the PL Macro- 

 cephalus. 



The sternal ribs, or abdominal haemapophyses, in the present fossil, are partially brought into 

 view on the left side of the abdomen : they seem to be relatively slenderer than in the PL Haw- 

 kinsii, and are consequently more like those of the PL Dolichodeirus ; as in both these species they 

 are connected by an intervening mesial, transverse, sternal bone, of a rib-like form, but pointed at 

 both extremities. This is the representative of the haemapophyseal spine, which is here, in re- 

 lation to the expansion of the abdominal cavity, developed in breadth or transversely, instead of 

 longitudinally or vertically, as in the tail of fishes. 



Pectoral Extremity. 



Of the bones composing the sterno-clavicular arch, we easily recognise the broad, expanded 

 coracoid, of which the humeral or articular extremity is seen projecting on the left side of the 

 fossil. 



This extremity is relatively broader than in the PL Harvhinsii, being adapted to a broader 

 joint at the corresponding extremity of the humerus. The notch above the articulation, which 

 receives the humeral end of the clavicle, is shallower than in the PL Hawkinsii. 



The rest of the apparatus is too imperfect to allow of an accurate comparison being carried 

 further. 



In the bones of the paddle, or radiated appendage of the pectoral arch, the following differences 

 exist between the PL Macrocephalus and PL Hawkinsii. 



The humerus is (as has been just observed) less contracted at its proximal extremity: it is 

 altogether less curved backwards, the anterior margin being concave instead of. straight or con- 

 vex, as in PL Hawkinsii. The distal end terminates in a slight but regular convex curve; while 

 in tlie PL Hawkinsii the separate facets for the radius and ulna are distinctly marked, and meet so 

 as almost to form an obtuse angle. 



In the Plesiosaurs generally the radius is nearly straight, while the ulna is bent, with the con- 

 cavity towards the radius ; both bones are flattened, as in the other Enaliosaurians. In PL Ma- 

 crocephalus the margin of the radius next the ulna is more concave than in PL Hawkinsii, and the 

 bone is relatively broader at its distal extremity, which is terminated by a convex instead of a 

 nearly straight line. 



The ulna equals in length the radius, as in the PL Dolichodeirus, while it never attains the same 

 length in the PL Hawkinsii. It is, in the Macrocephalus, relatively broader than in either the 

 PL Dolichodeirus or Hawkinsii. It also presents a more complete reniform figure ; the humeral 

 articular surface not being so straight, or so distinctly marked off from the outer convex margin. 



