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RAYMOND: ISOTELUS GIGAS DEKAY. 259 
be difficult to distinguish pygidia of [sotelus gigas, I. maximus, and 
I. 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 J. cowensis or I. maximus, 
but if the ratio of length to breadth is above .65 it is probably I. cowen- 
sis, and if below, J. maximus. In all cases, however, one must be 
dealing with adult specimens, and must take all characteristics into 
account. With isolated young 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 Proétus spurlocki (Pal. Ohio, 1873, 1, p. 161, 
pl. 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 I. 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 Jsotelus gigas has eight. Both have long cheek spines, small 
pygidia, rather long cephalons, and narrow axial lobe. The ratio of 
length to width in the cephalon of the “ Proétus”’ is rather high, (.69), 
for a specimen of Jsotelus gigas of this size, but it is also much higher 
than one would expect in I. maximus. The smallest specimen in the 
collection which is surely identifiable as [sotelus 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 Proétus spurlocki which have been found at 
Cincinnati seem to have all come from the Eden shale, where Jsotelus 
maximus seems to be more common than I. 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 I. gigas. Being, for 
the genus, a relatively primitive form, it reaches its specific habit 
quite early, and the principal variations among the specimens more 
