The Ventral Armor of Dinichthys. — Wright. 319 
(d) The less breadth of the posterior plates behind than in 
front. 
The foregoing facts seem to furnish a good basis for the 
rearrangement of the ventral plates of Dinichthys. It is to 
be regretted that we are not yet able to bring the ventral 
armor into positive connection with the cephalic or dorsal 
armor, or with the position of the pectoral fins. The anterior 
ventro-lateral plates have their outer margins rabbeted for a 
certain distance, along which they were probably sutured to 
some other plate not yet identified. A large number of plates 
are in the hands of collectors and museums, for the correct 
location of which no sufficient clues have yet been found. 
Concerning the structure of the bones represented in the 
plate, it may be well to explain that figures 6 and 8 represent 
the inner aspect of the respective bones, and the radiating 
lines show the direction of the grain or fibrous structure, 
which is more distinctly seen upon the inner than upon the 
outer surfaces. Fig. 8 represents a magnificent anterior lat- 
eral, the largest and most perfect, I think, which has yet been 
found. Several cross-sections of it are given to show the 
thickness and the structure of the border. In figure 3 (and 9) 
a remarkable device is seen by which it interlocks with figure 
4 in such a manner as to prevent longitudinal displacement. 
A strong, transverse, tooth-like ridge stands out on number 3 
which is received into a corresponding channel in the over- 
lapping number 4. In number 7 (which corresponds to num- 
ber 4) it would seem that the tooth to be received must have 
been much weaker. This device gives one more bit of evi- 
dence as to the turbulent life which was lived b} T these tyrants 
of the Paleozoic seas, and of the force of the attacks against 
which they were provided by their interlocking armor. 
EXPLANATION OF PLATE IX. 
Figures 1 to 4 represenl the paired bones belonging to the ventral 
armor of u single specimen of Dinichthys terrelU Newb., in their Datura! 
relal ive positions. 
Figs. 1 and :l. Right and lefl anterior ventro-lateral plates. 
Figs. 3 and 1. Righl and lefl posterior ventro-lateral plates. 
Pigs. 5 and 7. The left anterior and posterior ventro-lateral plates of 
another and larger individual, in their natural relations. The dotted 
lines ai the anterior end of fig. 5 denote five verj shallow, transverse 
channels, on the inner surface of the plate, which indicate the imping- 
ing border of a median ventral plate. 
