RAYMOND: ISOTELUS GIGAS DEKAY. 263 
in collection of J. E. Narraway, Esq.), from Faribault, Minnesota, 
in collection of the Carnegie museum), from Platteville, Wisconsin, 
nd from Newport, New York, have been compared, and measurements 
aken to determine the ratio of the length of the glabella to the total 
ength of the cephalon. It was found that the glabella was relatively 
horter on young specimens than on older ones, although there was 
ot much difference. A cranidium 7 mm. long has the glabella .78 
f the total length, while a specimen 62 mm. long has the glabella .88 
fthelength. The width of the brim is therefore relatively only about 
alf as great in the old specimen as in the young one. On the adult 
pecimen of A. wisconsensis in which the brim is widest, it occupies .19 
f the length, and on the specimen on which it is narrowest it occupies 
nly .12, the average being .15. On the type of A. romingert it occu- 
ies only .07 of the length, and on the only other specimen of this sort, 
; occupies .11. With the exception of these two specimens, cranidia 
rom all the localities conform closely to the general average. 
The agreement in proportions and ribbing among the pygidia 
rom the localities just mentioned is remarkable. The proportion 
f length to width is quite constant in specimens above 20 mm. long, 
nd is usually about .60. The ribbing on the larger specimens is 
nly a little less strong than on the small ones, and all have the same 
umber of ribs and rings. The largest pygidium in the collection is 
rom Platteville, and is 64 mm. long and about 105 mm. wide. 
