588 



PKOF. H. G. SEELET ON THE 



the last four terminate in the same transverse line, showing that the 

 vertebrae behind them are lumbar. The massive cylindrical character 

 and small head are the most distinctive features of the rib. 



Delicate, curved, flattened, fusiform ossifications extend trans- 

 versely over the ventral interspaces between the ribs, as well as 

 beneath some of the ribs, and beneath the centrums of some 

 vertebrae. These are the abdominal ribs. They are five or six times 

 as numerous as the costal ribs, and are composite, joining each other 

 by squamous overlap or contact, so as to form a ventral armour 

 like that seen in the abdominal ribs of certain Plesiosaurs, with 

 a median riblet and two lateral elements on each side. The 

 middle bones are 7 millim. long. 



No. 2. — 49971. Presented by Maurice Marcus, Esq., from Mining 

 Board Cutting, 50 feet deep ; 100 feet from the eastern margin of 

 Kimberley Mine, March 1878. 



This is a natural mould of the dorsal region which exposes the 

 dorsal aspect of the corresponding portion of a similar but slightly 

 larger animal ; for while seven vertebrae in No. 1 measure 4'7 centi- 

 metres, seven vertebrae in this specimen measure 5*5 centimetres. 

 But there is no difference in the character of the vertebrae and ribs. 



Eight vertebrae with their ribs are exposed in sequence, and the 

 flattened filamentous abdominal ribs are seen between the costal 

 ribs, admirably preserved- This armour is quite unlike that attri- 

 buted to Mesosaurus tenuidens, and may indicate another species. 



The neural arch, seen from above, has a wide subquadrate form ; 

 it is 6 millimetres long and 1 centimetre wide, but the width becomes 

 less in the hinder vertebrae. It is slightly wider in front than 

 behind. The dorsal surface is divided into two lateral subhorizontal 

 areas, which are convex from back to front, by a very thin neural 

 spine, which was vertical, compressed to a sharp edge anteriorly and 

 posteriorly, and appears to have been low. 



Laterally the neural arch of each vertebra gives off a strong 

 tubercle. It is subcorneal, compressed from above downward, 

 placed well below the level of the neural platform, towards the 

 anterior end rather than in the middle of the side of the vertebra. 

 These tubercles extend the width of the arch for 2 or 3 millimetres 

 on each side in the earlier vertebrae, but they appear to be slightly 

 shorter and slightly lower in position in the last vertebrae preserved. 



The dorsal ribs lie in natural sequence. At first sight they appear 

 to be wider proximally than in the other specimen, because they are 

 exposed so as to show the superior convex dorsal curvature ; but 

 enough of the margin is exposed to show that the proximal end was 

 compressed from above downward, so that there is no difterence of 

 condition, though these ribs are uniformly wide from the proximal 

 to the distal end. 



At the hinder extremity of this slab are two early caudal vertebrae, 

 isolated, and partly exposed ; one showing the anterior, the other 

 showing the posterior articular face. 



The anterior aspect displays the relatively small size of the face 

 of the centrum as compared with the neural arch, the width of the 



