754 THE AMERICAN NATURALIST. [VoL. XXXII. 
their thin edges preserved entire, thus showing the full extent 
of the overlapped area. We know this condition very well for 
the element in question, but the same cannot be said for the 
antero-dorso-lateral, whose thin overlapped edges are invariably 
broken away in all species. Accordingly, we have had to fol- 
low only the broken margin in drawing the outlines of this 
plate in Figs. 2 and 3, although without doubt the area over- 
lapped by the dorso-median on the one side and clavicular on 
the other was much greater. The union of the dorso-lateral 
plates in this species is one of simple overlap, and not by pegs 
and sockets, as in D. ¢errelli and D. intermedius. Here again 
the resemblance is with Coccosteus. 
Through an oversight, the markings on the antero-dorso- 
lateral, where the overlapping clavicular came in contact with it, 
were omitted from the diagram, but are essentially as shown 
in Fig. 3. One example of the clavicular was obtained by Mr. 
Teller near Milwaukee. It is an extremely heavy plate, bifur- 
cated anteriorly as in other species and tuberculated on its 
external surface. 
Another point of resemblance to Coccosteus is observed in 
the slight anterior emargination of the dorso-median. The 
exposed area between this plate and the occiput was very small 
in comparison with other Dinichthyids. As shown in Fig. 2, 
the forward part of the dorso-median is quite destitute of 
tubercles, and the demarcation of the barren area takes place 
along an oblique line extending from the middle of the shield 
toward either of the antero-external angles. This we regard as 
indicative that the anterior portion of the plate was buried 
beneath the integument. The median keel on the inferior 
surface is well developed and terminates in a massive posterior 
process, which attains a length of over 12 cm. in the adult. 
It depends at a greater angle with the shield than in the larger 
species, being in fact almost vertical. In this respect it agrees 
with other primitive tuberculated forms, such as D. livonicus, 
D. trautscholdi, D. pelmensis, and presumably also D. ringue- 
bergi. The average length of the shield in the median line, 
exclusive of process, is about 21 cm. in the adult, and maxımum 
width about 36 cm. 
