38 
the majority are so much compressed that their transverse 
diameter considerably exceeds their length in the direction of 
the axes of the ridges to which they belong. 
As remarked by Mr. Barkas, the tooth thus described is 
distinguished from the teeth of the most nearly allied species, 
C. cristatus, by the comparative smoothness of the inner moiety 
of each ridge, and by the distinct separation of the much 
compressed denticles. That it is a comparatively constant form 
of tooth in the Carboniferous Limestone Series of South 
Scotland, has been pointed out* to the present writer by Dr. 
R. H. Traquair, F.R.S., during a recent visit to Edinburgh ; 
and Ctenodus interriiptus is doubtless to be regarded as the 
Lower Carboniferous representative of the well-known C.cHstatiis 
of the Coal Measures. 
A. Smith Woodward. 
ON TWO ORODPS OF TEETH OF THE 
CRETACEOUS SELACHIAN FISH PTYCHODUS. 
Several discoveries of large groups of teeth of the well-known 
Cretaceous genus, Ptychodus, have already been recorded from 
the English Chalk; and in the case of one species, P. decurrens, 
the dentition has been found in a sufficiently complete state for 
satisfactory restoration.* The fact has also been emphasised, 
that these associated groups of teeth prove the ornamentation 
of the dental crown to have varied little in different parts of the 
mouth. Hitherto, however, series of figures of such associated 
teeth have only been published in the case of Ptychodus 
decurrens, and, to a slight extent also, of P. multistriatus f and 
one variety of P. poly gyrus. + It is thus of interest to be able, 
through the generosity of William Reed, Esq., F.Gr.S., to offer 
a few remarks on two fine groups of teeth of other species 
or varieties in the Society’s Museum, illustrated by the 
accompanying Plate I., figs. 3—20. 
* Smith Woodward, “ On the Dentition and Affinities of the Selachian Genus 
FtychodmP Q,uart. Journal Geol. Soc., vol. xliii. (1887), p- l‘^3, PI. x. ; also 
‘‘ Synopsis of the Vertebrate Fossils of the English Chalk,” Proc. Geol. Msoc., 
vol. X. (1888), p. 296, woodc. 
t Smith Woodward, “ Catalogue of Fossil Fishes in the British Museum,” 
pt. i. (1889), p. 146, PI. V., tigs. 4—6. 
