RAYMOND: ISOTELUS GIGAS DEKAY. 263 



(in collection of J. E. Narraway, Esq.), from Faribault, Minnesota, 

 (in collection of the Carnegie museum), from Platte ville, Wisconsin, 

 and from Newport, New York, have been compared, and measurements 

 taken to determine the ratio of the length of the glabella to the total 

 length of the cephalon. It was found that the glabella was relatively 

 shorter on young specimens than on older ones, although there was 

 not much difference. A cranidium 7 mm. long has the glabella .78 

 of the total length, while a specimen 62 mm. long has the glabella .88 

 of the length. The width of the brim is therefore relatively only about 

 half as great in the old specimen as in the young one. On the adult 

 specimen of A. wi^conscnsw in which the brim is widest, it occupies .19 

 of the length, and on the specimen on which it is narrowest it occupies 

 only .12, the average being .15. On the t}T)e of A. romingeri it occu- 

 pies onl}^ .07 of the length, and on the only other specimen of this sort, 

 it occupies .11. With the exception of these two specimens, cranidia 

 from all the localities conform closely to the general average. 



The agreement in proportions and ribbing among the pygidia 

 from the localities just mentioned is remarkable. The proportion 

 of length to width is quite constant in specimens above 20 mm. long, 

 and is usually about .60. The ribbing on the larger specimens is 

 only a little less strong than on the small ones, and all have the same 

 number of ribs and rings. The largest pygidium in the collection is 

 from Platteville, and is 64 mm. long and about 105 mm. wide. 



