RAYMOND: ISOTELUS GIGAS DEKAY. 259 



be difficult to distinguish pygidia of Isotelus giga-f(, I. maximus, and 

 /. iowensis, but such is not the case. If the pygidium has straight 

 sides, it can quickly be placed as I. gigas. If the posterior end is 

 rounded, it might be taken for either /. iowensi'} or /. maxitnus, 

 but if the ratio of length to breadth is above .65 it is probably /. iowen- 

 sis, and if below, /. maximum. In all cases, however, one must be 

 dealing with adult specimens, and must take all characteristics into 

 account. With isolated yoimg specimens, it is not always possible 

 to state to what species they belong, as the specific characters are not 

 assumed until late in life. A case in point is the young specimen 

 described by Meek as Pro'etus spurlocki (Pal. Ohio, 1873, 1, p. 161, 

 pi. 14, fig. 12). 



Proetus spurlocki Meek. 



The type of this species (Plate 1, fig. 3) which is in the Dyer collec- 

 tion in the M. C. Z., is clearly a young Isotelus, but whether it is the 

 young of Isotelus gigas or of /. maximus, the writer is not able to decide. 

 The specimen is 8.5 mm. long, and when compared with a specimen 

 of the same size from Trenton Falls, the only apparent difference is 

 that the specimen from Cincinnati has only seven thoracic segments, 

 while the Isotelus gigo^ has eight. Both have long cheek spines, small 

 pygidia, rather long cephalons, and narrow axial lobe. The ratio of 

 length to ^N-idth in the cephalon of the "Proetus" is rather high, (.69), 

 for a specimen of Isotelus gigas of this size, but it is also much higher 

 than one would expect in /. maximus. The smallest specimen in the 

 collection which is surely identifiable as Isotelus maximus is consider- 

 ably larger than the type of P. spurlocki, being 16 mm. long. In this 

 specimen, the length of the cephalon is .62 of the width. This is 

 slightly above the average for adult specimens, where this ratio ranges 

 from .57 to .60. 



The specimens of Proetus spurlocki which have been found at 

 Cincinnati seem to have all come from the Eden shale, where Isotelus' 

 maximus seems to be more common than /. gigas, and on that ground 

 the presumption would be that this specimen belongs to the former 

 species. It seems very probable, in any case, that the young of the 

 two species would be alike at this stage of development. 



Changes during the life history of Isotelus maximus. — Isotelus maxi- 

 mus seems to be a much less variable form than /. gigas. Being, for 

 the genus, a relatively primitive form, it reaches its specific habit 

 quite early, and the principal variations among the specimens more 



