THE 
GHOLOGICAL MAGAZINE. 
NEWRSERIESa | DECADE “Nil “VOLES OC 
No. X.—OCTOBER, 1893. 
(QyisvItGAlIN( AE VAs Svar washS\- 
I.—Some Cretacrous Pycnopont Fisuss. 
1.—On ArHRovoN. 
By Artuur Smita Woopwarp, F.L.S., F.G.S. 
(PLATE XVI.) 
T is well known that the crushing teeth of the Pycnodont fishes 
as a rule are arranged with striking regularity upon the bones 
which bear them, the vomer in the upper jaw and the splenials 
in the lower jaw. They are disposed in longitudinal series, gradually 
increasing in size backwards, and there is only one genus satisfac- 
torily known (Mesodon) in which there is any marked tendency 
towards the frequent disturbance of this linear arrangement. 
Thirteen years ago, however, Dr. Sauvage referred to the Pycno- 
donts some very robust splenial bones on which the teeth exhibited 
scarcely any regularity either in size or arrangement, and more 
specimens have subsequently been discovered which seem to justify 
his determination. For the fish to which these bones belonged, 
Dr. Sauvage proposed the generic name of Athrodon; and two 
species were described from the Portlandian and Kimmeridgian 
formations of the north of France. 
That Athrodon does pertain to the family of Pycnodontide and is 
generically distinct from any of the satisfactorily determined forms 
may now be regarded as certain. So far as the present writer has 
been able to observe, the Pycnodonts never exhibit successional 
teeth ; when the fish is young the teeth on the splenial and vomer 
vary little in size and are comparatively irregular; when the fish 
grows the bone extends and widens backwards, the series of teeth 
rapidly assuming their regular and distinctive characters. The 
splenial bone of Athrodon also exhibits no successional teeth, and it 
is thus apparently to be regarded as a genus of Pyenodonts, in which 
the initial irregularity of dental arrangement persists more or less 
throughout life. There is a tendency towards the differentiation of 
a principal longitudinal series of teeth in some species; and in a 
general way the number of series that would be present, if there 
were less irregularity, can be counted. The splenial bone, however, 
does not differ from that of other Pycnodonts merely in its dentition; 
it is more robust than in other genera, and the roughened symphysial 
facette is at least twice as deep as usual. In some specimens, indeed, 
the dentition is perhaps insufficient to determine whether they per- 
DECADE IlI.—VOL. X.—NO. X. 28 
