138 
Annals of the Transvaal Museum. 
piece of the same specimen a small portion of the tail, adjoining the 
sacral vertebra, is preserved (PL XVII). This shows four caudals without 
chevron bones. Another specimen (PI. XIX, fig. 2) shows a good many 
caudals, seven of which do not have chevron bones. The seventh, how- 
ever, has already got two knoblike, processes, which further backwards 
develope into chevrons. Whether the eighth had distally coalesced 
chevrons cannot be definitely made out, as they are broken off near the 
hypocentrum. In the ninth, tenth and eleventh vertebra they are also 
broken off, but their base is so well developed that I presume they were 
distally united. Their base also reaches from the front to the hinder 
border, therefore over the whole length of the hypocentrum ; this is not 
the case in the seventh and eighth vertebra. In the seventh the knoblike 
processes are situated near the hinder border of the hypocentrum, and in 
the eighth the base of the processes only nearly reaches its front border. 
Probably, therefore, the eighth caudal did not have distally coalesced 
chevrons. The tail of Myriodon did then have at least thirty-six caudal 
vertebrae, and may have had forty-six. 
The longitudinal ridges on the lower surface of the first six hypo- 
centra are better developed than in the presacrals ; otherwise they show 
no marked differences. The angle of the chevron bones as also of the 
neural spines becomes less towards the end of the tail. Pleurocentra are 
still present in the thirteenth caudal of the large piece and up to the 
ninth in the other. In this specimen (PI. XIX, fig. 2) the left pleuro- 
centra of the eighth and ninth consist of two small elements instead of 
one. There is only one large right pleurocentrum in the eighth while 
that of the ninth is missing. 
The neural spines of the caudal vertebrae are each thicker in front 
than behind. Proximally they are, like those of the presacrals, narrower 
at the base, but further distally they acquire a more uniform breadth. 
As the chevron bones, the spines grow gradually smaller towards the end. 
The anterior and posterior zygapophyses are still normal and in touch 
with each other in the 26th caudal. Further towards the end they do not 
reach each other any more. The two bony elements of each vertebra in 
this region are far apart from each other and from those of the next 
vertebra, the whole indicating the presence of a great amount of cartilage. 
Ribs. 
All the presacrals, as far as present, have ribs. They are present 
on the caudals up to the ninth vertebra. They are all single-headed, 
that is to say, although there is really only one proximal end, two 
articulation facets can easily be distinguished. These facets make an 
obtuse angle with each other, their adjoining border protruding slightly 
from the bone. The tubercular portion is thick and rounded, while the 
capitular portion is flatter and thinner. The lower surface of the rib 
between the tubercular and capitular portions is strongly concave. The 
shape of the ribs has been greatly changed through fossilization. (From 
some it can be seen that they were bent, thin in the middle, and that their 
distal ends were broad and flat. There are indications that the more 
forward ribs produced a processus uncinatus from the proximal part of 
this flat portion. 
The single sacral rib is large, and its head, though much stronger, is 
formed in exactly the same way as the other ribs. The two large facets 
are still in contact with the diapophysis and the large articulation surface 
of the hypocentrum on either side. The rib is slightly constricted near 
