Section IV, 1889. [ 113 ] Trans. Eoy. Soc. Canada. 



X. — Some Remaries on the Classijication of the Trilobites, as ivfluenced hy Strati- 

 graphical Relations : with Ontline of a New Grouping of these Forms. By 

 E. J. Chapman, Ph.D., LL.D., Professor in the University of Toronto. 



(Read May 8, 1889.) 



(1.) It may be taken as an undovibted fact that palœontological classification has 

 been very greatly influenced of late years by stratigraphical considerations. This is 

 seen not only in the current subdivisions of the Trilobites, but in those also of the 

 Ammonites and other extensive groups. These stratigraphical classifications have the 

 advantage of being readily effected. They save trouble, by requiring little or no thought 

 for their construction, and they are of course useful to the geologist as palœontological 

 lists : but there their value ceases. Structural affinities become by this plan more or less 

 unrecognized ; and forms vrith but few characters in common, if occurring at the same 

 geological horizon, are thus often forced into false relationship, rendering even moderately 

 rigid definitions of families and other groups practically impossible. 



(2.) It might be thought, in opposition to this vievs^, that contemporary forms of a 

 given order or family must be more nearly related to each other than to forms of the 

 same order or family occurring at earlier or later x)eriods. But this conception is cer- 

 tainly in the main erroneous. Forms of the same geological horizon should naturally offer 

 fewer points of generic agreement than forms of different horizons. The latter may be 

 connected by more or less direct evohition : whereas forms of the same horizon can only 

 be related generically, if at, all, through some remote ancestral type, from which, also, 

 other distinct orders and classes may have sprung. In one case, there may be direct 

 relationship : in the other, the connection can be little more than indirect. 



(3.) An impression prevailed widely at one time, and perhaps still prevails, that the 

 so-called " Primordial Trilobites " are distinguished from the Trilobites of higher horizons 

 by a combination of characters peculiar to themselves, by w'hich a marked " primordial 

 aspect " is imparted to them. These characters, as commonly formulated, comprise : — A 

 large, typically horned or spiny head-shield, wiih numerotis body segments and a very 

 small pygidium. This deiiuitiou faiLs, of course, completely in the case of the eminently 

 Cambrian family, the Agnostidte. But setting aside these still somewhat problematical 

 forms, and looking only to the typical Trilobites, it is found to be equally inapplicable in 

 many other cases. Whilst, for example, it holds good in Paradoxides, Oleuus, Eurycare, 

 and some few other Cambrian genera, we find the same combination of characters — the 

 large and horned head-shield, the long thorax, the small pygidium — present also in 

 Harpes, an essentially Upper Silurian and Devonian genus, unknown in Cambrian strata. 

 The imaginary primordial aspect is sufficiently well marked in the Cambrian Oleuus ; 

 but in the Silurian and Devonian Arethusina or Aulacopleura we see a combination of 

 characters very similar to those of Olenus, among which may be specially cited the 

 comparatively short glabella, spiny head-shield, open facial suture, small eyes connected 



Sec. IV, 1889. 15. 



