Apr. 1903. NorTH AMERICAN PLESIOSAURS—WILLISTON. 67 
That the species described in the foregoing pages belongs in 
some genus named but not recognizably described from a later epoch 
is not probable, though possible. I have therefore given the genus 
the name Arachauchenius. 
POLYCOTYLUS LATIPINNIS. 
PADDLE.—Some years ago an excellent specimen of a paddle of a 
plesiosaur belonging in all probability to Polycotylus latipinnis Cope, 
was collected by Mr. George R. Allman of Wallace, Kansas, from the 
upper Niobrara chalk of the Smoky Hill river, east of Fort Wallace. 
The bones of the paddle were, for the most part, found in their 
‘ natural relations, but were separated in the collection of them. The 
radius and ulna of a second paddle, together with some of the smaller 
bones showed weathering, and doubtless had been picked up from the 
surface. It has required but little trouble to fit into their natural 
relations all the bones except most of the phalanges, which, present- 
ing no lateral surfaces for articulation, could only be located from 
their other characters. A careful study of these, however, makes it 
probable that the positions assigned to them in the photograph are 
for the most part correct. Because of the considerable expansion 
distally of the long bone, the paddle is supposed to be the front one, 
but it is quite possible it may be a hind one. Pl. XXI. 
The head of the Awmerus is large and broad, of a flattened ellip- 
soidal form, with the surface nearly evenly convex; it is slightly 
crushed dorso-ventrally. The tuberosity, placed at a slight distance 
beyond the plane of the proximal extremity, is massive, It has two, 
large, flattened, narrowly separated surfaces for muscular attachment, 
placed nearly at right angle to the longitudinal plane of the bone 
and separated from the head by a slight groove on either side, that of 
the ulnar side being the more pronounced. On the ulnar side of the 
tuberosity there is a slight rugosity, as though for muscular attach- 
ment. The shaft is narrowest near the upper third of the bone, 
where a cross-section would be nearly circular, or slightly greater in 
its dorso-ventral diameter. The anterior or radial border is gently 
convex on its upper two-thirds, gently concave below to the rectan- 
gular angle. The posterior border is more deeply concave through- 
out. The distal border has a deep, cupped, thickened facet at right 
