568 CARBONIFEROUS LABYRINTHODONT 
The best-preserved rib is 6! inches long?! and half an inch broad, 
measured in a direction perpendicular to its length. It is, however, 
much flattened from before backwards, so that its thickness does not 
amount to more than one-sixth of an inch. The face of the rib is not 
flat, but it is somewhat excavated, so that the bone is thinner in the 
middle than at the edges. At its proximal end the rib exhibits a 
very distinct cvberculum and capitulum. The former projects, so as to 
disturb the sweep of the curve of the convex side of the rib and to 
convert it for a short distance into a concavity, and it is abruptly 
truncated posteriorly. The capitulum of the rib continues the line of 
its general curvature for half an inch beyond the tuberculum, and 
ends in a rounded extremity. I presume that the capitulum 
articulated with the lower half of the transverse process (c), and that 
the tuberculum articulated with its upper half, in which case the 
distance (a, 2) on the rib would be practically equal to the excess of 
length of the upper division of the transverse process over that of the 
lower. 
The skull, a dorsal vertebra, and a rib of Axthracosaurus being 
known, I now return to the question, what are the affinities of that 
Labyrinthodont ? 
The large size of the teeth, the comparative solidity of their 
bases, and the complex character of the labyrinthic ramifications of 
the pulp-cavity are all characters in which Axthracosaurus resembles 
the J/astodonsaurus of von Meyer and Plieninger and its allies, and 
differs from Archegosaurus. Whether Anthracosaurus had well-ossified 
occipital condyles like Jfastodonsaurus, or cartilaginous ones such as 
were probably possessed by Archegosaurius, does not appear, the fossil 
being defective in this region. In the large size of the anterior 
palatine foramina, the extent to which the palate-bones are united 
with the mawxillaries, in the form of the pterygoidean arch and that of 
the basisphenoid, Axthracosaurus is nearer to Archegosaurus and 
Dasyceps than to Mastydonsaurus. 
But the vertebre are altogether Mastodonsaurian. The vertebre of 
Mastodonsaurus were described and figured in 1844 in the well-known 
work of von Meyer and Plieninger, ‘ Beitrage zur Palaontologie 
Wiurtembergs.’ No fewer than seventeen vertebre were discovered in 
one slab, together with the skull of this remarkable Labyrinthodont ; 
another block contained eight vertebra, belonging to the same animal, 
but not immediately succeeding the former ; and a third slab of stone 
' The sternal end of the rib is broken off. It was certainly much longer when perfect, 
as the rib from Lord Enniskillen’s collection, though more slender, measures 84 inches 
along its curve, and still presents a fractured extremity. 
