NO. 24 PTARMIGANIA STRATA — RESSER 6l 



genera representing this sort of trilobite shows that here is another 

 genus that formerly would simply have been called Ptychoparia. It 

 resembles several genera, such as Elrathia, Elimaniella, Clap pas pis, 

 and many others. Its primary distinguishing feature, so far as general 

 appearance goes, is its plumpness, brought about by convexity in both 

 directions. When flattened down by photography, Pachyaspis appears 

 most like Ehmaniella, with which I do not think it is related. 



The glabella tapers slowly forward to a rounded front outline. The 

 dorsal furrow is clearly defined, but not deep. In side light the glabella 

 appears nearly smooth except for a slight keel, but in sharp longi- 

 tudinal light four pairs of furrows become visible. The rear pair is 

 recurved and very wide and seems to branch. The remaining pairs 

 run about straight across the glabella. A slightly irregular occipital 

 furrow, which is stronger than the dorsal furrow, separates a ring 

 of nearly equal width throughout. This ring carries a small 

 nuchal spine. 



The brim consists of a slightly convex preglabellar area, which 

 slopes down from the dorsal furrow. A clear but shallow anterior 

 furrow separates a nearly flat, slightly upturned rim, which is a little 

 more than half as wide as the preglabellar area. 



The fixigenes average about half the glabellar width. They are 

 convex to almost the same degree as the glabella. Forward of the 

 eye the fixigene widens by the slight divergence of the facial suture. 

 Behind the eye normal posterolateral limbs are formed. 



The eyes are small. They are situated at about the midpoint of the 

 cranidium. Eye lines of ordinary strength extend back to the eyes 

 beginning near the anterior end of the glabella. 



In spite of the abundance of specimens the libragene has not been 

 determined. Neither is the pygidium known. No doubt it is small 

 and inconspicuous and therefore may be confused with something else. 



Name. — Tragi's = plump ; ao-iris = shield. 



Genotype. — P. typicalis, new species. 



PACHYASPIS TYPICALIS, n. sp. 



Plate 11, figs. 15-20; plate 12, figs. 1-3 



The generic diagnosis and illustrations adequately present specific 

 characteristics. 

 Locality 54s. 

 Holotypc and paratypes. — U.S.N.M. Xos. o,8564a-f. 



