A NEW FORM OF “DIPLOCAULUS” 55 
the group passed. The large, strong vertebrae; the well-developed 
spines, and the highly curved ribs approach the normal condition 
of the more generalized Stegoceph. ‘The well-developed zygosphene 
and zygantrum articulations bespeak a strength and flexibility 
of the vertebra column such as belongs to a creature that has 
developed swift progression through the water by means of tail 
propulsion. 
The remarkable pit development on the spines of this form 
suggests a still further step in its modifications for swift movement 
through the water. It seems highly probable that the pits formed 
an articulation for anteroposteriorly movable spines that gave 
support to a dorso-median fin. The lateral rigidity of such a 
fin was assured by the double articulation indicated by the well- 
developed double pits. As would be expected, the anterior fin 
spine gives evidence of its superior size and strength of articu- 
lation through the very conspicuously larger pit on the second 
spine. The present specimen lacks the first vertebra, but the 
several strings of previously known forms examined by the writer 
establish this point. In the backward folding of the fin spines 
extra space would be required behind the enlarged anterior spine. 
Perhaps this accounts for the uniform absence of the pit or pits 
in the third vertebra. : 
Not entirely in keeping with these swift-swimming modifications 
is the remarkable development of the shoulder girdle. The frag- 
ment that the writer identifies as the ascending process of the 
right clavicle is stout and something over 60 mm. long. It is 
strongly grooved on the outer-posterior side for articulation with 
the scapula. In all figured specimens of Dzplocaulus and the many 
other specimens examined by the writer this process is short. In 
some cases it may have been broken off but after making due allow- 
ances for this it does not seem likely that there was an actual 
articulation with the scapula or at best, but a weak one. Perhaps 
the ancestral Diplocaulus was a creature that had become well 
adapted to swift progression through the water to the extent of a 
suitable vertebral evolution and the reduction of the limbs. At 
a later stage this actively swimming form likely degenerated to 
the groveling type in which habitat the girdles continued to reduce 
