62 
DAVIS : FOSSIL FISH KEMAINS. 
extended edge forms a cutting surface which is toothed Hke a saw 
with the largest denticles in front and gradually diminishing in 
size backwards. The tooth of the lower jaw is devoid of 
denticles ; it is raised in the centre of the external edge in the 
form of a single point, which fits a corresponding hollow inside 
the margin of the upper tooth. The pair of teeth present a 
resemblance to the beaks of a parrot, in the manner in which they 
fit to each other. 
Amongst the fishes of the primary or secondary rocks there 
is no other genus known which approaches in form and character 
to Pristodus. Amongst living fishes the Diodonts offer a some- 
what close resemblance to it. Diodon is an inhabitant of tropical 
seas ; the fishes are sometimes called "sea hedgehogs," they are 
covered with a thick skin without scales, over which are distribut- 
ed a large number of spines. They have the power of inflating 
the body, and assuming a globular form their spines become 
extended, like those of a hedgehog. Their jaws consist of a single 
undivided plate, and are admirably adapted for breaking off 
branches of corals, from which they principally obtain their food. 
Fossil remains of Diodon have been found in the tertiary lime- 
stones of Monte Bolca. The Pristodus of the Carboniferous 
Limestone agrees with the recent Gymnodont Diodon in its 
dentition, but there in all probability the resembla,nce ended. 
The recent fish has a bony skeleton and is covered with bony 
spines, the fossil was apparently cartilaginous, and no spines have 
been found which at all resemble those of Diodon. It is very 
likely that the similarity of the dentition may indicate a similarity 
of diet, the remains of coral are abundant in the limestone. 
In addition to the genera of fossil fish -remains already men- 
tioned and which have been previously described, there are others 
which have not received the same attention, but these in all 
probability may be included in one or other groups, already 
indicated. The peculiar spines, Cladacanthus and Physonemus, 
