THE STRUCTURE AND RELATIONSHIPS OF ANCODUS. 471 
it is correlated with increased length and curvature of the neck. That Ancodus 
should be added to this list of parallelisms is obvious from the comparison of the 
process in the species from the lower beds with those from the upper sandstones. 
In the American Museum of Natural History, New York, is preserved a frag- 
mentary skeleton of A. amen canus, from, the Metamynodon-beds (or middle Oreodon- 
beds of the White River), for an opportunity to study which I am indebted to the 
kindness of Messrs. Osborn and Wortman. It is in many ways different from the 
skeleton of the large A. brachyrhynchus, which forms the principal subject of the 
present description. In A. americamis the neck is shorter than in A. brachy- 
rhynchus and the axis especially is different ; it is smaller and, in particular, shorter 
(33 : 46) with lower, but relatively wider atlanteal surfaces, which present more 
directly forward. A more important difference is in the character of the odontoid 
process, which is conical, though slightly depressed, the transverse diameter some- 
what exceeding the dorso-ventral. The ventral face of the process is flattened and 
much less strongly convex than the dorsal, which is the reverse of the shape in the 
later species. The odontoid has an upward as well as a forward direction and is 
longer and more slender than in the pigs. 
The posterior face of the centrum is wide, depressed, concave, and oblique in 
position. The neural arch is broad and high, enclosing a large canal, a:nd the neural 
spine resembles that of Agriochcei'us, forming a great plate with curved and slightly 
thickened border; its anterior end projects over the atlas in the form of a blunt 
hook. The postero-superior angle is broken away in the only available specimen, 
preventing the determination of its exact shape, but the thickening at this point 
indicates a posterior rib-shaped prolongation, such as occurs in Dicotyles and Oreo- 
don, though not in Agriochoerus. The postzygapophyses are large and prominent 
and present directly downward. The transverse processes diverge widely from the 
centrum, projecting outward more than backAvard, but with their free ends recurved 
somewhat toward the median line. These processes are obscurely trihedral in 
shape and are proportionately heavy; they are perforated by the vertebrartenal 
canal, and their anterior ends are connected by low ridges with the articular facets 
for the atlas. 
The other cervical vertebra; have moderately elongate and heavy, opisthocoe- 
lous centra, with obliquely set faces; they have very faintly marked ventral keels 
and the liypapopliysial tubercles are either rudimentary or altogether .absent. Ot 
the cervical series the 7th is the shortest, and next to that the 6th. In relative 
length the neck considerably exceeds that of and even that of Oreoim n 
the case of the latter genus the comparison cannot fairly be made with the sku 1, 
on account of the extreme shortening of the face, which in Ancodus is as extremely 
elongated. But Ukiiig the humerus as a standard of comparison, the neck is pro- 
portionately longer in Ancoiia brackyrhymhm than m Oreodm. The neural 
Lhes are broad and low, and the pedicels of the neural amhes are not perforated 
for the exit of the spinal nerves, as they are in the pigs. These pedicels have very 
little antero-posterior extent, while the tygapophyses are very prominent; those 
