EASTMAN: THE DINICHTHYIDS. 23 
plates of Dinichthys must be separate or fused in all members of the 
genus.” 
Under ordinary circumstances, such an interpretation would appear 
most logical, since we should expect, a priori, marked differences in the 
mode of union of the mid-ventrals to be indicative of different genera. 
We might reasonably infer that these differences were accompanied by 
variations in the dentition and other parts of the body, although this is a 
point which could only be determined empirically. Should it be ascer- 
tained, however, that forms existed having a like dentition, a like con- 
figuration and arrangement of plates as in Dintchthys, yet differing among 
themselves as respects the mode in which the median ventrals were 
united, there would be difficulty in estimating the value of this latter 
character. Ought it to be regarded as a valid generic distinction, or, 
other things being equal, merely as an adaptive variation: affecting 
different species indiscriminately? From present indications it would 
appear highly probable that diverse conditions existed in the ventral 
plates of forms which agree in their remaining characters, so far as 
known, with Dinichthys. It must be noted, also, that amongst the 
species of this genus the paired ventral plates are exceedingly variable 
in their characters, more so in fact than any other plates of the body. 
Not only do they vary in form, relative proportions, and mode of union 
among different species, but there are considerable differences to be ob- 
served within one and the same species; one class of variations within 
specific limits will be referred to later under the head of ventro-lateral 
plates. 
To sum up these difficulties briefly, we must admit on the one hand 
that theoretical considerations are opposed to the view that species of 
one and the same genus should differ widely as respects the number and 
arrangement of the median ventrals; but on the other hand, evidence 
is wanting to show that the forms they represent differed in any respects 
further than this from Dinichthys. And until positive evidence is forth- 
coming, such as finding the plates naturally associated with the denti- 
tion, it is impracticable to employ characters of the ventral plates as a 
test of generic rank. In our opinion, both prudence and convenience 
dictate that plates which resemble the known elements of Dinichthys, 
when found in the detached condition, are to be referred to that genus 
until criteria are at hand for determining them otherwise. Accordingly, 
certain isolated plates, whose description follows, will be referred to 
Dinichthys by virtue of their obvious affinities with that genus. And 
it will be assumed, provisionally at least, that in this genus the median 
