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 detei'mined empirically. Should it be ascer- 

 tained, however, that forms existed having a like dentition, a like con- 

 figuration and arrangement of plates as in Dinichthys, 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. 

 Kot 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 ai'e 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 aflBnities with that genus. And 

 it will be assumed, provisionally at least, that in this genus the median 



