138 PALEOZOIC FISHES OF NORTH AMERICA. 
plates. The first pair are smooth on the outer surface, about half an inch 
in thickness, the under side nearly smooth but somewhat radiately marked, 
as most of the plates of Placoderms are. Along their inner margins they 
overlap, so that the points are brought near together. By a singular and 
ingenious device they are prevented from slipping on each other by a deep 
notch in the edge of the upper plate, which surrounds and is filled by a cor- 
responding conical ridge rising from the surface of the under plate ; by this 
key they were firmly locked together. The posterior pair of jugulars—or 
as they should perhaps be called, hyoid plates—are long-triangular in out- 
line, smaller than the anterior pair, but much thicker. Their anterior angles 
overlap and are sunk into the obliquely truncated ends of the jugulars. 
The outside and posterior ends of the hyoid plates are irregular and thin, 
and show that they were overlapped by other plates. 
I have been led to conclude that the four plates just described covered 
the under portion of the head, because they were plainly on the median 
line, and when in apposition formed a shield, which had the proper outline, 
and would nicely fill the otherwise defenseless area between the mandibles 
and anterior to the plastron. It is evident that while so flat and so firmly 
locked together they would not be adapted to the protection of the posterior 
part of the body behind the plastron or the dorsomedian plate, the after part 
of the body requiring more flexibility than they would permit. 
No figure is given of the plates which are supposed to have formed 
the posterior half of the plastron, because no perfect ones have been found, 
but I have numerous fragments of relatively large plates which must have 
been oblong in form and had the moderate and uniform thickness and 
plainness of surface which characterize the plates that defended the under 
side of the body. As they are apparently assignable to no other place in 
the armor of Dinichthys I provisionally locate them here. 
Dr. Traquair, in his important paper published in the Geological 
Magazine of January, 1889, calls the suborbital bones of Coccosteus the 
maxillaries, and that would seem to be the most natural reading of the 
anatomical structure. But in Dinichthys Terrelli the dental armature of the 
upper jaw consists of a cleaver-like plate, of which the lower cutting edge 
played on the upper margin of the mandible like the blades of shears; a 
