REMARKS ON PETALODUS ALLEGHANIENSIS 175 
from limited localities in Yorkshire and Armagh. Attention 
has been called repeatedly to the great differences existing 
among detached teeth belonging to one and the same species, 
due to conditions of wear, age, geographical distribution, and 
other causes; and particularly to the wide range of variations 
displayed by teeth of the same individual, depending upon the 
position occupied in the mouth. Inthe case of Petalodus, we 
can infer what this second class of variations were, from analogy 
with the closely related genus Janassa. All of the minor differ- 
ences in form and size which our author observes between P. 
securiger and P. alleghantensis may be reasonably referred to this 
category. 
The marked variation in the size of P. alleghaniensts correlat- 
ing with geographical distribution has already been commented 
on by Newberry (Pal. Ohio, Vol. II., p. 53), the Ohio specimens 
being only about one-half as large as the western ones. Had 
our author compared his specimen with the figures and descrip- 
tion of Leidy’s type (Jour. Acad. Nat. Sci. Philad. [2], Vol. 
III., p. 162), instead of with the one figured in the Extinct 
Vertebrate Fauna, he would have found that the agreement is 
closer than he supposes; among other features, the number of 
basal enamel folds is precisely the same in both forms. 
Finally, the rounding of the lateral angles (only one is pre- 
served on Dr. Hay’s specimen), can hardly be considered as of 
specific importance, since it, too, is a variable function; nor does 
the prominence of the enamel bands on the posterior surface 
appear to have any particular significance. The conclusion 
reached by the present writer is that at the most P. securiger is 
only a variety of P. alleghaniensis. 
As to the availability of the terms P. extinctus and P. ohioensis, 
both are clearly untenable. Leidy’s provisional designation of 
“ Stcarius extinctus,”’ proposed in 1856, was merely tentative, and 
unaccompanied by either figures or description of any kind. 
When, a few months later, the species was adequately described, 
this name was withdrawn, as was entirely proper, and that of 
Petalodus alleghaniensis substituted instead. In all probability 
