EASTMAN: THE DINICHTHYIDS. 21 
gest and most perfect plate that the writer has seen is preserved in the 
Museum of Comparative Zodlogy, and measures 65 cm. in length (Cata- 
logue No. 1325). ‘The corresponding element in D. intermedius is hardly 
to be distinguished except for its smaller size. An excellent example of 
the latter species belonging to the School of Mines Cabinet of Columbia 
University shows the postero-dorso-laterals of either side of the body 
commingled with other plates pertaining to the same individual; it is 
valuable for furnishing comparative measurements of the different bones, 
and deserves further study. 
The orientation of the plate in question may be readily determined, 
either by an inspection of the overlapped area, or by noting the course 
of the sensory canals. These arise at the anterior border, where they 
meet the single straight furrow that traverses the antero-dorsal-lateral ; 
and from this point they sweep inwardly, sometimes as a single and 
sometimes as a double channel, as far as about the middle of the exposed 
portion of the plate, where they cease. In this respect the genus differs 
froin Coccosteus, which has the canals continued on to, and in some 
cases entirely across, the dorso-median. The insunken area formed by 
the overlap of the:latter plate stands in marked contrast to the irregular 
depressions produced by the overlap of the antero-dorso-lateral. The 
graceful curve forming the postero-lateral boundary of the dorso-median 
is projected upon the underlying plate, and shallow depressions are left 
where the transverse ridge on the under surface of the dorso-median 
rested on the subjacent plate. This ridge, it should be noted, occupies 
the same relative position as its homologue in Coccosteus. 
The upper boundary of the lateral plates is indicated by a deeply in- 
sunken area on the antero-dorso-lateral, and a slight indentation on the 
free margin of the postero-dorso-lateral. Below, these as yet undiscov- 
ered plates must have been connected with the ventral armoring, either 
directly, or more probably through the intervention of the “claviculars.” 
The curvature of the ascending arm of the latter furnishes us at the same 
time with the curvature of the missing laterals, and we can also form 
an approximate estimate of their height and length. It is to be hoped 
that the laterals may yet be identified as such, when the entire dermal 
covering of Dinichthys can be compared plate for plate with its European 
congeners. 
Ventral Plates. — Grave difficulties have been encountered in the 
attempt to reconstruct the ventral armor of Dintehthys, owing to the 
detached condition in which the plates have invariably been found. Tt is 
perhaps but natural that the views which were originally entertained 
