RAYMOND: ISOTELUS GIGAS DEKAY. 



255 



IsoTELrs 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. 



^3 



Fig. 2. — A very small specimen of Isotelus gigas, exposed from the imder 

 side. This specimen shows the rounded cephalon and pygidium and 

 divided hj-postoma, though it is too poorly preserved to show how many 

 thoracic segments are present. X 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 /. gigas. Natural size. M. C. Z. 

 Nos. 48, 38-41. 



Isotelus iowensis Owen. 



Plate 2, fig. 6; Plate 3, fig. 1, 2. 



Isotelus iowensis, Owen, Rept. Geol. sur. Wise, Iowa, Minn., 1852, p. 577, pi. 

 2a, figs. 1-7. Clarke, 1897, Pal. Minn., 1897, 3, 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 



