Apr. 1903. Norra AMERICAN PLESIOSAURS—WILLISTON. 63 
length between that of the eleventh and of the thirteenth. On the 
last cervical centrum, the thirteenth, the twelfth rib is long, heavy 
and stout, more than half the length of the first dorsal. It is nearly 
as stout as any of the following, but tapers somewhat distally, though 
ending in a truncated, cartilaginous extremity. The distal end of the 
eleventh rib is lost. The first dorsal rib resembles the one preceding 
it, though longer. The second dorsal rib has acquired nearly the full 
length of the thoracic series. In the ninth rib there is a beginning of 
a diminution in size; the rib is less thick, a little shorter, and less 
expanded at its extremity. The twelfth rib is completely preserved; 
it is yet smaller and thinner than the eleventh, though-still possessing 
a cartilaginous extremity. Of the following ribs, only the heads of 
some are preserved. 
DiaPopHyses.— The diapophyses occupy an unusually low position 
on the arch of the whole dorsal series, as do also the cervical ribs. 
They have not been wholly freed from the matrix, and their relation 
to the articular process is determinable only in slight part in a few of 
the posterior vertebre. The arch, like the cervical ribs, is united by 
a strong, persistent suture, evidently an adult character, since the 
sutures of the atlanto-axial complex have been entirely obliterated. 
The process of the first dorsal 1s short, compressed, and somewhat 
expanded at each extremity; it clearly springs from below the dorsal 
surface of the centrum. As already described, the last cervical rib, 
but little shorter than the first dorsal rib, is attached to a short 
process which arises, apparently wholly, from the centrum. The 
diapophysis of the first dorsal is less than twice the length of this 
process, and so much like it that its sutural connection with the 
centrum is the chief distinctive difference. The second dorsal dia- 
pophysis is a little stouter than, and about twice as long as the first. 
Its articular surface for the rib is larger, flattened, and looks down- 
ward and outward. The fourth and fifth processes have attained the 
maximum size of the series. They have a narrow, concave ventral 
border, more strongly concave posterior border, a flattened, expanded 
proximal end for union with the centrum and an expanded distal 
extremity with its flattened oblique costal surface. The height of the 
process distally, and its expansion beyond the plane of the articular 
zygapophyses, can not be determined. It is very evident, however, 
that the diapophyses he below the plane of the zygapophyses, in 
which they differ markedly from the diapophyses of Dolichorhyn- 
chops, where, throughout most of the dorsal vertebra, they are placed 
wholly above the plane of the zygapophyses. The succeeding dia- 
