NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES IO9 



that the segments were nine in number, rather wide, and grooved as 

 in Asaphus. 



" Associated p,ygidia referred to the genus semi-elhptical to tri- 

 angular, always much wider than long, with a fairly distinct axis that 

 may or may not reach the margin, and in others prolonged posteriorly 

 into a spine. Segmentation imperfect, usually obsolete except for 

 one or two ribs at the anterior edge, or with as many as four segments 

 on the axis but never more than two on the pleura, that latter being 

 for the greater part smooth. Surface gently convex to the edge or 

 with an obscurely defined, wide, marginal depression. Doublure 

 rather wide, about as in Asaphus." 



. Dimensions. — The known species range in length from less than 

 one inch to perhaps three inches (2.54 to 7.62 cm.). 



Genotype. — Symphysurina luoosteri Ulrich. 



Stratigraphic range. — Lower or Middle to Upper Ozarkian. 



GeograpJiic distribution.— Onthtc, vipper Mississippi Valley, Ne- 

 vada, Alberta and British Columbia. 



Observations. — " As defined Symphysurina is remarkable in that 

 it includes species with and others without genal spines ; also species 

 in which the axis of the pygidium fails to reach the posterior margin, 

 and others in which it is prolonged posteriorly in a long spine. In 

 some the pygidium has a flattened or concave border and in others 

 the slope of the pleural lobes continues convex to the edge. Such 

 differences are often regarded as of generic significance, but in 

 Symphysurina the cranidial characters are so constant that they do 

 not appear to be of more than specific importance. Besides, the asso- 

 ciated pygidia of the various species exhibit among them every gra- 

 dation from the condition obtaining in S. illaenoides, in which the 

 end of the axis is blunt and one-fourth its length from the margin, 

 to the long-spined condition observed in .S'. spicata and 5". goniura. 

 The progressive chain formed by the pygidia of S. illaenoides, 

 S. billingsi, S. striatifrons, S. eurekensis, S. obtusa, S. zvoosteri, 

 S. spicata and S. goniura, while probably not altogether genetic, is 

 yet too gradual in the differentiation of its limits to encourage one 

 in drawing even a subgeneric boundary between any two of them. 

 And it is much the same with respect to the genal spines and the sub- 

 marginal grooves. Apparently Symphysurina affords here merely 

 another illustration of the unstability of the Ozarkian trilobites in 

 characters that were developed more regularly in the preceding Cam- 

 brian and succeeding Canadian and later periods. 



" Two species of this new genus have been referred to Sym- 

 physurus by Brogger and Raymond, the former having done this 



