No. 382.] DINICHTHYID OSTEOLOGY. 753 
originate below the middle of the central plates and sweep 
inwards and downwards not far in advance of the median 
occipital element, sometimes even traversing it for a short 
distance (Fig. 1). Similar isolated canals occupy the same 
position inthe crania of Titanichthys (Fig. 4), and very often 
a reminiscence of them appears in D. éerre//z. 
The canals traversing the external occipitals form a Y, whose 
descending branch passes across the articulating condyle of 
the antero-dorso-lateral, and thus emerges upon the dorsal 
system of body plates. In Coccosteus the canal traversing 
the antero-dorso-lateral bifurcates as soon as it crosses the 
condyle, a branch running toward either of the posterior angles 
of the plate, and that running toward the postero-internal 
angle is continued upon the dorso-median plate. In Dinich- 
thys and Titanichthys there is no such bifurcation on the 
antero-dorso-lateral, but the canal is single, extending back- 
ward along approximately the middle of the plate, and thence 
on to the postero-dorso-lateral. Nevertheless, in D. pustulosus 
a reversion toward Coccostean conditions is occasionally met 
with, inasmuch as the antero-dorso-lateral may have a second 
short canal, ending blindly, as shown in Fig. 2. 
None of the American Dinichthyids have heretofore been 
known to have the dorso-median traversed by sensory canals, 
although this condition exists in a small European species, 
described as D. pelmensis.1 But the dorso-median of D. pus- 
tulosus bears distinct traces of canals, albeit the grooves are 
narrower and shallower than those of the antero-dorso-lateral 
plate. They extend obliquely backward from the point where 
they leave the postero-dorso-lateral and terminate just before 
reaching the median line of the shield. Only one example of 
the postero-dorso-lateral has thus far been encountered,? and 
as it lies with its external surface embedded in the matrix, the 
course of the canal system across this plate (indicated on the 
diagram by dots instead of dashes) has yet to be verified. 
Plates that are evenly embedded like this are likely to have 
1 Bull. Mus. Zool., vol. xxxi (1897), Pl. II, Fig. 4. 
2 Now deposited in the Milwaukee Public Museum with the rest of Mr. C. E. 
Monroe’s private collection, of which it forms a part. 
