NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES III 



the same genus. But neither of these species has the essential charac- 

 ters of Symphysurus. Both fall definitely into the new genus Sym~ 

 pJiysurina. 



" Taking into account both the hypostoma and the dorsal surface 

 of the trilobite, the relationship of Symphysurina to Platypeltis 

 Calloway ^ is perhaps as close as to any other established genus. The 

 differences lie in the cephalia of the two genera, especially in the 

 course of the facial sutures and consequent distinctions in the forms 

 of the cranidia, free cheeks, and eyes. Judging from the form of 

 the cranidium and the general shape and construction of the cephalon, 

 Platypeltis represents a younger stage in the evolution of the asaphid 

 trilobites than is represented by Symphysurina. 



" One other British genus may be mentioned as probably related 

 to Symphysurina, namely, Psilocephalus Salter, based on a Lower 

 Tremadoc species. Critically compared, the latter proves to have a 

 relatively much larger cranidium, with broader fixed cheeks, this 

 difference being particularly notable in the posterior limbs. The 

 course of the facial suture in Psilocephalus also is quite different, 

 the curves both in front and behind the eyes, as shown by the outlines 

 of the cranidium, being decidedly convex instead of nearly straight 

 t© distinctly concave. 



" Of American genera, Platycolpus, proposed a few years ago by 

 Raymond ^ for species of the type of Bathyurus capax Billings, agrees 

 in certain respects closely with Symphysurina, but the margin of the 

 palpebral lobes in Platycolpus is less convex, especially in the anterior 

 half, and is provided with a raised rim. Another constant difference 

 is to be noted at the anterior edge of the cranidium. This is bordered 

 by a simply thickened, generally finely striated rim in Symphysurina, 

 whereas in Platycolpus the rim expands into an oblique or vertical 

 flattened coarsely striated area. 



" As a rule the dorsal furrows are more distinct and the glabella 

 correspondingly better outlined in Platycolpus than in Symphysurina. 

 But a more striking difference is noted in comparing the posterior 

 parts of the cranidia. In Platycolpus, namely, the neck furrow and 

 neck ring are usually more or less clearly developed, which is never 

 the case in Symphysurina. The postero-lateral limbs also are much 

 nearer the genal angles than in Symphys-urina. Another important 

 difference is in the location of the small median tubercle which is 

 found on the glabella between the eyes in Symphysurina and on the 

 neck ring in Platycolpus. 



* Quart. Journ. Geol. Soc, Vol. Z3, 1887, p. 664, pi. 25, figs. 2, 2a. 

 Victoria Mem. Mus. Bull. No. i, 1913, p. 62,. 



