Distribution of the Rhabdophora. 191 



a few major divisions of wide extent and vague limits. In 

 both we find these larger sections subdivided into a series of 

 minor groups, more rigidly defined, separable in their turn 

 into still smaller sections of strata, or grouplets of life-forms, 

 the comparative importance of which varies within certain very 

 small limits dependent upon local convenience or personal 

 equation. In both, however, if we eliminate all the contro- 

 verted matter and have respect merely to acknowledged facts, 

 enough remains to satisfy us that many of the grander out- 

 lines of our subject are even now clearly discernible. Future 

 investigation must take origin from our present standpoint. 

 While, therefore, much will long remain a matter for indivi- 

 dual opinion, it may be expected that future research will 

 tend mainly to give clearness and completeness to our present 

 schemes of classification, repairing their deficiencies by the 

 intercalation of new members, and substituting the definiteness 

 of exact knowledge for the vagueness of partial ignorance and 

 provisional opinion. 



Of the imperceptible but ceaseless growth of this clearer 

 knowledge we have an instance in the present study, which 

 has served to bring insensibly into prominence many facts 

 pointing to a more convenient grouping of the Rhabdophora 

 themselves than that hitherto in use among palaeontologists. 

 Having regard to the most probable alliances and geological 

 distribution of the various genera of the Rhabdophora recog- 

 nized to this date, we find that they are most satisfactorily 

 classified for the purposes of comparison in four main groups, 

 each of which is composed of an association of allied genera, 

 in which the polypary is constructed of a fundamental and 

 special element or elements variously combined. The first 

 group may be said to consist of simple and complex forms of 

 the genus Monograptus ; the second of variously modified 

 forms of the biserial genus Diplograptus ; the third of simple, 

 complex, and compound modifications of the bilateral genus 

 Didymograptus ; and the fourth of similar variations of the 

 genus Dicellograptus. 



The most circumscribed of these groups (Monograpta) is 

 composed solely of the family of the Monograptidse. The 

 great systematic importance of this family clearly entitles it 

 to divisional rank. It is the only section of the Rhabdophora 

 in which all the component genera possess both unilateral 

 and uniserial polyparies. To this section Hopkinson's title 

 of Monoprionidse should in future be restricted. 



The second group (Diplograpta) , which is founded on 

 Diplograptus as a type, and includes the three families of the 

 Diplograptidse, Lasiograptidse, and Retiolitidse, has the 



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