37 
ON THE TOOTH OF A CAEBONIFEEOHS 
DIPNOAN FISH: CTEN0DU8 INTERRUFTUS. 
The known remains of the Upper Palaeozoic Dipnoan Fish, 
Ctenodus, are all of so fragmentary a nature, that the teeth alone 
can, at present, he employed for the distinction and definition 
of the species of the genus. It is thus unfortunate to observe 
that these teeth not only vary considerably in the different 
stages of growth of individuals of a single species,* * * § hut are also 
much changed in the appearance of their dental crown by various 
agencies of post-mortem corrosion and abrasion. According 
to the latest researches, several supposed “ species ” owe their 
distinction merely to causes of this nature, and the table of 
synonymy is hence somewhat lengthy.f Moreover, a few 
forms that appear to be correctly regarded as well-marked 
specific types, are as yet only inadequately defined; and the 
present note relates to a tooth in the Society’s Museum which 
falls under the latter category. This fossil was obtained 
from the Carboniferous Limestone Series of Grilmerton, near 
Edinburgh, and is shown, of the natural size, in Plate I-, fig 2 ; 
it was briefly noticed by Mr. T. P. Barkas, F.H.S., in 1869, J 
under the name of Ctenodu8 interrupt us ^ and a short diagnosis, 
without figure, was published later in the same year.§ 
The tooth is referable to the left side of the lower jaw, and 
is somewhat broken at the margins, especially in its anterior 
half. There are indications of fourteen coronal ridges, of which 
the foremost (x) is broken, bent forward, and much abraded, 
while the eighth and eleventh appear as intercalated at the 
outer margin, not extending inwards beyond the middle of the 
tooth. So far as preserved, each complete ridge is subdivided 
in its outer half into a series of broad distinct tuberculations, 
but becomes much laterally compressed in its inner half, and is 
even or exhibits only few feeble traces of transverse constrictions. 
The tubercles have the form of large rounded mammillae, and 
* A. Fritsch, “Fauna der Gaskohle, etc., Bohmens,” vol. ii. (1888), p. 71, PI. 
Ixxiii. 
t Woodward and Sherborn, “ Catalogue of Briti«b Fossil Yertebrata” (1890), 
pp. 51 — 53. 
X “ Scientific Opinion,” vol. i., p. 515. 
§ Ibid., vol. ii., p. 113. 
