Section IV, 1880. [ 1^3 ] Teans. Roy. Soc. Canada. 



X.—Somc Remarl-s on the Cla^'sification of the Trilobites, as influenced Inj Strati- 

 (jraphkid Relation.s : toith Outline of a New Grouping of these Forms. By 

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



(Kead May 8, 1889.) 



(1.) It may be takon as an undoubted fact that pal:voutologioal classification has 

 been very greatly iuiiuenced 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 paloeontological 

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

 unrecognized ; and forms with 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 ai:d other groups practically impossible. 



(2.) It might be thought, in opposition to this view, 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 periods. But this conception is cer- 

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

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

 connected by more or less direct evolution : whereas forms of the same horizon '-an 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 conueition 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 iharacters peculiar to themselves, by which a marked " primordial 

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

 large, typically horned or spiny head-shield, wilh numerous body segments and a A'ery 

 small pygidium. This definition fails, of course, completely in the case of the eminently 

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

 forms, and looking only to the typical Trilobites, it is foxmd to be equally inapplicable ui 

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

 and some few other Caml^rian 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 suiiiciently well marked in the Cambrian Olenus ; 

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

 characters very similar to those of Olenus, among whi<'h may be specially cited the 

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



Sec. IV, 1889. 15. 



