318 The American Geologist . November, 1894 
plate was fully 30 inches in length, a length greater by six 
inches than that of any mandible known. It is, of course, 
possible that the jugular may have exceeded the mandible in 
length, but we know that the mandibles of Dinichthys were 
capable of the freest and most vigorous action. 
In the large "jugular" just referred to, the curvature of the 
outer margin was strong, and similar to that of figs. 3 and 4 
of the plate, where, with a length of 16 inches, we have a 
breadth (of the pair) of 14 inches. The curvature here is far 
greater than that of any mandible which I have ever seen. 
The curvature of the mandibles, indeed, is very slight. The 
single "jugular," however, of which Dr. Newberry figures the 
inner and outer aspects,* is of the type shown in fig. 7, where 
the curvature of the outer border is much reduced. It is 
aside from my present purpose to discuss the question whether 
the two types of plates here shown belong to the same species 
or not. I will only remark, in passing, that the plate figured 
by Dr. Newberry is referred by him to D. terrelli; while those 
shown in figs. 3 and 4 of the present paper are known to be- 
long to I), terrelli by the structure of the dentition upon 
which the species was founded, and which was preserved 
along with the plates in question. 
As we consider, therefore, the large size and the strongl} T 
curved border of the plates which have been called "jugulars," 
we may be the more willing to see them transferred to the pos- 
terior portion of the ventral armor, where they are freer from 
limitations as to size and shape. 
4th. This rearrangement of the plates brings the "plastron" 
of Dinichthys into more striking agreement with that of Coc- 
costeus (fig. 1 in text) than did the first reconstruction by Dr. 
Newberry. To the harmonies which he pointed out,f the fol- 
lowing may be added : 
(a) The overlapping of the posterior plates by the extremi- 
ties of the anterior plates. 
(b) The overlapping of the right posterior plate by the left 
posterior plate along the median line. 
(c) The sinuous line of overlapping between the posterior 
plates in both genera. 
*Monograph, PI. vi. 
fOhio Geol. Survey. Paleontology, vol. ii, p. 9. 
