FROM THE YOREDALE SERIES. 629 
one by a well-defined, but narrow and somewhat deep concavity. 
The under surface of the tooth is concave, and conforms generally 
in outline to the convex surface of the crown. 
Teeth of lower jaw (Plate X XVI. fig. 13) more or less oblong in 
outline, about the same length as those of the upper jaw, but 
considerably narrower. Surface composed of three ridges as in 
the teeth of the upper jaw; they are more acute, and separated 
by wider interspaces. The central one is extended obliquely from 
the inner to the outer margin ; it is prominent, rounded, and con- 
siderably worn by attrition, especially midway between the inner 
and outer margins, where the ridge is considerably raised above the 
parts before and behind. ‘The anterior ridge defines the margin of 
the tooth ; it is well-developed and straight. The posterior ridge'is 
small, and very obliquely extended, giving to the posterior extremity 
of the tooth a pointed character. The under surface of the tooth is 
concave, and exhibits the characteristic convolution peculiar to the 
rou 
: These teeth are distinguished from those of Deltoptychius acutus, 
Ag., by their smaller size and thinner and more delicate form ; 
they are more regularly oblong and the posterior extremity of the 
tooth of the lower jaw is considerably less pointed than in the type 
species. The teeth of the upper jaw are broader and more expanded 
on the outer margin, and the ridges of the crown are more promi- 
nent, In all these particulars the present species differs still more 
from Deltoptychius gibberulus, Agass. 
Genus Psammopvus, Agassiz. 
PsawMopus Rucosus, Agass. Poissons Fossiles, vol. iii. p. 111, pl. xii. 
fies. 14-18; pl. xix. fig. 15 (1833). 
One or two teeth of this genus have been found; they are com- 
paratively small. ‘The finest example is slightly less than an inch 
in length, and 0°3 inch in breadth. ‘The root is thick and massive. 
The margins of the crown of the tooth along its longer axis are 
prominent and rounded, the central portion hollowed, Coronal 
surface worn, and covered with coarse punctures. 
The teeth of Psammodus are much smaller than those found in 
other localities, in the limestone occupying a much lower horizon ; 
and whilst they are similar in every other respect to the larger teeth 
of the Mountain Limestone, they appear to indicate that the genus 
was gradually dying out. In the Millstone-grit beds sya. it 
appears to have disappeared entirely. 
Genus AstRABopts, Davis, g. n. 
Teeth of medium size, saddle-shaped, prominently raised near one 
end, diminishing in breadth and thickness very rapidly anteriorly, 
much more gradually posteriorly to less than half the greatest 
Q.J.G.8. No. 160. 20 
