EASTMAN: THE DINICHTHYIDS. 25 
associated or not with other remains cannot now be ascertained. Theo- 
rotical considerations are certainly opposed to the idea that this plate 
pertained to either D. terrelli or D. ingens ; and we are compelled to 
regard it as indicating an as yet unknown Dinichthyid species. 
Fused Median Ventrals. — Under this head must be placed the two 
examples already referred to, that have been described by Newberry * 
and Dean.? The originals are preserved in the School of Mines Cabinet 
at Columbia University, and have been determined as D. terrelli and 
D. (1) gouldi. Whether fusion took place as a strictly adaptive charac- 
ter in forms having a thin plastron, whether it occurred only in adult 
individuals, or whether it characterized all the individuals belonging to 
particular species, are questions for future discoveries to determine. 
That fusion did not exist in all species of Dinichthys appears, however, 
extremely probable. 
Overlapping Median Ventrals. — Species which have the postero-ventro- 
median overlapped by the anterior element represent the normal or 
primitive condition, as exemplified by the genus Qoccosteus. Three 
specimens of the detached antero-ventro-median and two of the postero- 
ventro-median are preserved in the Cambridge collection, whose relations 
to contiguous plates were plainly those of overlap and underlap. © The 
bone shown in Plate 2, Figs. 5, 6, exhibits such a striking resemblance 
to its homologue in D. terrelli, that there can be no doubt as to its iden- 
tity. It is evident that the plate under discussion is entire, since its mar- 
gins taper gradually to a thin edge, and show no signs of having been 
broken away from a lower portion. Hence, the only important difference 
that is to be observed between this specimen and D. terrelli relates to the 
mode of union with the posterior element ; in the present case it overlaps, 
in D. terrelli it is fused with the hinder piece. As we know of no other 
species to which it can be referred, we must include it, provisionally at 
least, under the last named species. 
The special characters of this plate have been described elsewhere, 
although at that time the specimen was supposed to belong to Titan- 
vchthys. It may be remarked in passing that the semicircular flange 
forming the anterior margin (seen best on the ventral aspect) is contin- 
uous with similar compressed borders on the antero-ventro-laterals. 
None of these margins reveal any trace of plates overlapping them in 
1 Report Geol. Survey of Ohio, Vol. II. Part. I. (Paleontology), pp. 10, 31, 
and Chart VI. Fig. A. 
2 Trans. N, Y, Acad. Science, Vol. XV. pp. 157-163, 1896; Ibid., Vol. XVI. 
Pp. 57-60, 1897. 
