476 ' THE AMERICAN NATURALIST. [VoL. XXXII. 
downward below the general level of the base, so as to provide 
a solid basis of attachment with its counterpart on the right 
side of the jaw. Distinct traces of this symphysial attachment 
are seen on the inner face in the form of a beveled band 4 mm. 
wide, running parallel with the front margin (see Fig. 28). 
There is no thickening on the inner face of the jaw at the 
symphysis, and in consequence the two upper dental plates 
must have been in close proximity to each other throughout 
the anterior portion of their length, in order to have permitted 
the beaks of the lower jaw to bite outside of them, which we 
know from other species must have happened. Although an 
excavated area appears on the outer face of the specimen 
behind the beak and below the cutting edge, yet this is plainly 
not due to wear, the boundaries being too sharply demarcated 
for that, and is therefore to be regarded as the natural configu- 
ration of this region in the present species. Other species, 
however, so far as known, have the outer face perfectly smooth, 
all traces of wear against the upper dental plate being confined 
to the inner side of the cutting edge. No upper jaws referable 
to this or any other European species have yet been encoun- 
tered, but detached tritors are not uncommon. 
(4) P. calceolus (Figs. 1-17).— This well-known species, 
the only one yet described from this country, is tolerably 
abundant in the Hamilton Limestone of the Upper Mississippi 
Valley and Manitoba ; but prior to the discovery of the State 
Quarry fauna, near North Liberty, Iowa, was never met with 
except in a fragmentary condition. The reason for this is 
because the tissue surrounding and in advance of the tritors is 
vascular and soft ; besides this the dental plates are consider- 
ably attenuated between the tritor and symphysial beak, and 
hence are extremely liable to be broken here through destruc- 
tive agencies. Detached symphyses occur in the State Quarry 
bed, to be sure, but are vastly outnumbered by separate and, 
for the most part, abraded tritors. Out of more than 5000 
specimens of Ptyctodus collected at this locality only about 
50 showed the symphysial region, and of these but four be- 
longed to the upper jaw. Approximately perfect dental plates 
are therefore of the utmost rarity. 
