520 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
this curving gradually upwards on each side towards the backwardly extended 
palate. The palatal surface is concave, its anterior margin is formed by a semicir- 
cular cutting-edge, the posterior margin extends with a shghtly inward flexure 
between the two latero-posterior extremities. The teeth consist of a thin bony 
shell, the internal surface conforming generally to the external one, and envelop- 
ing the cartilaginous jaws. There is no distinct base or root. The surface is 
thinly but uniformly coated with enamel, sraooth or slightly punctate. 
Messrs. St. John and Worthen, in the ‘Paleontology of Iiinois” (Vol. VI., p. 402, 
pl. Xa., fig. 6), refer a minute tooth, -09 inch in lateral diameter, to the genus 
Pristodus. Judging from the description and figure, the tooth has the serrated 
cutting edge, similar to that of Pristodus, as represented in the teeth from the 
Yoredale Limestones of Yorkshire, and it bears a further likeness in having no 
appreciable basal region. Leaving its small size out of consideration, it differs 
from Pristodus in several important particulars; it does not extend in a semicircular 
form backwards at the latero-posterior extremities of the tooth and has much more 
the appearance of having constituted one of a series of teeth in the shark-like types, 
or, aS is suggested by the authors, to one of those curious little fossils known 
under the general term Conodonts, than to the singular arrangement in Pristodus. 
In the latter there are always two central pronunences or cusps of the coronal surface, 
one on each side the median line, which are equal in size, whilst in the specimen 
from the Kinderhook fish-bed there is only one central prominence. The Yorkshire 
specimens are devoid of any imbrications or folds extending along the base of 
the crown; they are present in the P. acuminatus, St. J. & W. It is improbable 
that this tooth can be retained in the genus Pristodus. 
The teeth are probably unique in their characteristics amongst the fishes of the 
Palaeozoic rocks. A single tooth appears to have enveloped the whole of the jaw, 
upper or lower as the case may be. ‘Those of the upper jaw differed considerably 
from the lower. A strongly-serrated, trenchant, cutting-edge descended from and 
encircled the palatal portion of the crown. The serrations, having the character 
of a number of teeth or denticles anchylosed together, decrease in size backwards, 
the largest on each side the median line of the tooth occupying about one-third its 
vertical length ; broadly implanted they converge triangularly to an acute point. 
The tooth of the lower jaw is devoid of serrations and presents a surface, 
produced to form a single apical point in the centre, but otherwise plain and 
smooth. It is smaller than the upper tooth, and when the mouth of the fish was 
closed, the serrated portion of the latter enveloped the lower jaw extending far 
over its anterior surface, this is clearly shown by a specimen in the Collection of 
Mr. William Herne of Leyburn. The upper palatal portion of the jaw in this 
specimen is broken off, leaving the circular vertical part deeply serrated as usual 
and about a quarter of an inch in depth. ‘The iterior of the tooth is filled up 
with a matrix of limestone, and projecting from this is the pointed apex of the 
