No. 379.] DEVONIAN PTYCTODONTIDEZ. 479 
Just in front of the tritors the jaw is constricted, the inner 
face bending in close to the outer, which remains nearly straight, 
or may even curve slightly outward. Young forms have the 
constriction less marked than adults, since it becomes still 
further narrowed through wear. As already remarked, the 
outer face, which continues nearly vertical and smooth in ~ 
advance of the tritors and terminates superiorly in a knife-edge, 
clearly never came in contact with the opposing upper denti- 
tion, since all evidences of wear are confined to the inner face. 
Young forms have a relatively shorter cutting edge than full- 
grown individuals, indicating that the symphysis became pushed 
further forward with age. The cutting edge slopes rapidly 
upward in front and terminates in a sharp prehensile beak. 
Below, at the symphysis, there is a projection similar to that 
noticed in P. molaris, for the purpose of strengthening the 
symphysial attachment ; and the front margin joining these two 
projections is straight, or very nearly so, instead of convex as 
in the European species. The inner face of the symphysis is 
thickened and rounded in order to separate the rami sufficiently 
to close outside the upper dental plates. Two vertical lines 
sometimes appear near the front margin in well-preserved 
specimens, and include between them a wedge-shaped area 
having apparently a denser structure’ than the surrounding 
tissue. No beveling has yet been observed on any of the 
specimens to indicate a sutural union at the symphysis. Either 
such traces have been effaced by accident, or the dental plates 
were simply apposed and held in place by ligaments. Illustra- . 
tions of the symphysial region in different specimens are given 
in the accompanying figures. 
(5) P. compressus, sp. nov. (Figs. 18-27). — Besides P. cal- 
ceolus, two new species occur, although in lesser profusion, in 
the State Quarry bed, and one of them is found also in the 
Hamilton Limestone of Milwaukee. These new forms, which 
we will call P. compressus and P. ferox, are interesting on 
account of being transitional to the genera Rhynchodus and 
Paleomylus, respectively. The tritors in P. compressus are 
relatively narrower and longer than in P. calceolus, and between 
them and the symphysis a long, sharp, cutting edge is formed 
