On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 489 
and is divided into two portions, the anterior one occupies one-third the crown, 
it is a wide, hollow, arched, scoop-like cutting margin, which in some specimens 
is obscurely and minutely crenulated or denticulated, and is usually quite sharp, 
the second posterior part occupies two-thirds the crown, it is shield-formed, some- 
what convex, with the point directed backwards and the sides evenly arched 
outwardly. This portion is traversed by a series of strong transverse undulated 
ridges imbricated forwards and divided by wide deep grooves, where the ridges 
are much worn they are smooth, but where fresh they are rough or deeply 
notched. The root is a wide plate as broad as the tooth and tapers slightly 
backwards, behind it is rounded, convex above and concave below, and projects 
backwards on a lower plane, the crown being elevated above its upper surface. 
The lateral teeth are similar to the above, but they are inequilateral, one side 
concave the other convex. 
A comparison of the teeth of Janassa with those of Petalorhynchus exhibits a 
great degree of similarity and also many essential particulars of dissimilarity. 
The two genera agree in the quincunx arrangement of the teeth in the mouth, for 
notwithstanding the circumstance that no example showing the complete 
transverse dentition of Petalorhynchus has been discovered, still sufficient is known 
to prove that the teeth existed in transverse rows, and the probability is that the 
arrangement of the teeth in the two genera was uniform. That the vertical 
arrangement was the same in each is amply proved ; there is in each a gradual 
increase in size from before, backwards; the largest teeth being those in use, and 
the smaller ones serving to support and strengthen them. The process of growth 
or accumulation must have been the same in both species, each additional row 
being superimposed from the inside, a method which is almost unknown amongst 
existing species of sharks or rays. The two genera differ much in detail, however, 
in the form of the vertical sets of teeth ; in Janassa, the group of teeth form a 
thick mass, five or six teeth being equal in thickness to the length of the longest 
tooth, whilst in Petalorhynchus the teeth are thinner, and so arranged as to form 
a closed, dense mass, as firmly attached to each other as if they were anchylosed, 
and the breadth is less than one-third the length of the largest tooth. The 
coronal surface of the tooth in the two genera is divided into two parts, a 
petaliform portion for cutting, and a second imbricated palatal surface, but the 
relative importance of the parts is very different. The cutting surface of the tooth 
of Janassa is small and the apex is simply circular, that of Petalorhynchus large, 
longer than broad, and the central portion of the cutting edge, more or less pro- 
duced and acuminate ; in the latter also the imbricated surface is small or absent and 
does not appear to have served a very important part in the economy of the fish, 
and the long root has been deeply embedded in the cartilage of the jaw. The 
imbricated surface in Janassa is quite different, it occupies two-thirds of the crown 
or about one-half the entire length of the tooth, and it was in constant use for the 
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