650 PROF. H. G. SEELEY ON [A-Ug. 190O, 



ribs. On account of its extreme shortness the earliest rib, less than 

 2^ inches long, may belong to the neck. The fifteen which follow 

 may be counted as dorsal, and the four which succeed them may 

 be regarded as lumbar, the ribs being practically absent, as in 

 mammals, from the lower part of the back, where they do not 

 enclose the abdomen as in reptiles. 



The dorsal ribs gradually become longer, from about 4 inches in 

 the earliest to 6 or 7 inches at the tenth or eleventh ; but then they 

 diminish in length again, and the last dorsal rib is 5 inches long. 

 The transition is abrupt to the short ribs of the lumbar series. The 

 ribs of the back are strong, rounded from back to front, compressed 

 from side to side, with a shallow longitudinal groove running along 

 the posterior surface of each ; the anterior surface is not exposed. 

 Each rib diminishes in depth towards its extremity, but the width 

 remains unchanged. One specimen appears to show that the rib is 

 hollow or cancellous at the fractured extremity. The curvature of 

 the ribs is remarkably small, and in contrast with Plesiosaurs and 

 Ichthyosaurs, a character which appears to indicate that the cavitj T 

 of the thorax was deep in proportion to its width. There is no 

 trace of sternal or abdominal ribs preserved ; and Mesosaurus is 

 still the only South African reptile in which abdominal ribs are 

 known. 



The articular head of a dorsal rib shows no indication of sepa- 

 ration into two distinct facets. It is expanded a little from side to 

 side, convex or angular from above downward, and is produced 

 somewhat proximally. In the earlier ribs it is \ inch deep. 

 The effect of the form of the proximal rib-facet is to produce a small 

 e marginatum of the rib superiorly, external to the articulation, 

 which truncates the rib obliquely. Its upper part articulates in the 

 early dorsal vertebrae with the transverse process, and the lower 

 part articulates apparently with the centrum of the vertebra. The 

 two articular facets are not clearly seen on the side in any vertebra, 

 and it is possible that the lower or capitular part in the early 

 vertebrae may be between the centra, as in Cynognathus and 

 Mesosaurus. These ribs are generally similar to those of the small 

 Herpetocheirus brachycneinus, a Theriodont which I found at Klip- 

 fontein, near Fraserburg. 1 



The first lumbar or last dorsal rib is 3| inches long as preserved, 

 but may be an imperfect dorsal rib, being similar to the dorsal ribs 

 in position and curvature. But the four succeeding lumbar ribs 

 are each about 1| inches long, more cylindrical, uniform in character, 

 directed more transversely outward, a little curved at the proximal 

 end which is compressed from side to side, and deep as in the 

 dorsal region. 



The bodies of the dorsal vertebrae are very partially exposed. 

 They are at first less than \ inch long, and gradually increase in 

 length, so that the last dorsal or first lumbar centrum is | inch 

 long. At first the bodies of the vertebrae, as exposed in the large 



1 Phil. Trans. Koy. Soc. vol. clxxxvi (1895) B, p. 158 & fig. 4. 



