RAYMOND: ISOTELUS GIGAS DEKAY. 255 
IsorELus stage. The cephalon and pygidium both become smooth, 
and the axial lobe widens, as in the adult of Isotelus. 
IsOTELUS GIGAS stage. The cephalon and pygidium become tri- 
angular, and the spines are lost from the genal angles, a combination 
of characters distinguishing this species. 
Fig. 2.— A very small specimen of Isotelus gigas, exposed from the under 
side. This specimen shows the rounded cephalon and pygidium and 
divided hypostoma, though it is too poorly preserved to show how many 
thoracic segments are present. «10. M.C.Z. No. 36. 
Fig. 3-7.— The same species. A series of specimens showing the changes 
from rounded shields, narrow axial lobe, and long genal spines of the 
young to the pointed shields, wide axial lobe, and spineless cheeks of the 
adult. Fig. 3, 4 represent an Isotelus maximus stage, 5 and 6 an I. 
iowensis stage and 7 is as small a specimen as is often found showing all 
the characteristics of the adult of J. gigas. Natural size. M. C. Z. 
Nos. 48, 38-41. 
ISOTELUS IOWENSIS Owen. 
Pate. 2, hie. 6;.-Plate.3, :fig....1, 2. 
-Tsotelus iowensis, Owen, Rept. Geol. sur. Wisc., lowa, Minn., 1852, p. 577, pl. 
2a, figs. 1-7. Clarke, 1897, Pal. Minn., 1897, 8, pt. 2, p. 704. 
Although mentioned occasionally in faunal lists, this species seems 
to have been pretty generally neglected. It is so closely allied to 
Isotelus gigas, that, where the two species occur together, as at Trenton 
Falls, it seems almost like hair-splitting to recognize two species. 
Essentially, it is an Isotelus gigas which retains at maturity certain 
