706 THE PALEONTOLOGY OF MINNESOTA. 
[Isotelus gigas. 
The suggestions made here with reference to the morphological significance of 
the genal spines can be tested fully only when extensive series of specimens are 
brought under study. That in the subgenus Isofelus, they are infantile characters 
gradually eliminated in successive moultings of the test, appears to be true, not 
only of the individual, but of the race. 
IsoreLus ei¢as DeKay, 1824. 
Of late years the name introduced by Stokes, Asaphus platycephalus, for a 
trilobite from St. Joseph island, lake Huron, has become current for this species, 
on the ground of priority of description.* None of the figures given by Stokes show 
the structure of the genal angles, and it is therefore wholly a matter of presumption 
whether his specimens were of the same character as those afterwards fully described 
and illustrated by DeKay.+ 
Fig. 8.—Isotelus gigas DeKay. Hypostoma of a large individual. Hudson River sandy shales 
(erratic), central New York. 
Formation and locality of Isotelus gigas in the Minnesota formations. Hudson River group, Granger. 
There is a single nearly entire specimen which appears to have had a spineless cephalon, from the Galena 
limestone at Mantorville; and from the same locality a fragment of the glabellaof an immense individual, 
wliich in its entire condition must have had a length of not less than 17 inches. This is the largest 
- authentic specimen of an asaphid recorded, and I have here introduced an outline figure of the animal 
in its natural proportions. 
TsorELus MAximus Locke, 1838. 
This is the name first proposed by Locke, who subsequently changed it, for 
euphony, to J. megistos, under which it has usually passed. 
Formation and locality.—Trenton: Minneapolis, St. Paul, Rochester, Minn.; Mineral Point, Wis. 
Galena: Wykoff, Warsaw, Kenyon, Cannon Falls, Minn. Hudson River: Granger, Minn. 
*Trans. Geological Society, vol. i, 2nd Series, pp. 199, 208, pl. 27. 1822. 
+The Trenton rocks of New York contain a distinet species known at present only from its pygidium. This has passed 
under the name J. gigas, and is figured in the Paleontology of New York, vol. i, (pl. 61, figs. 3g, 3b), associated with cephala 
of corresponding size, but which may or may not belong to it. It is characterized by its broad, blunt, somewhat elevated 
posterior termination, and flat upper surface and axis. Notwithstanding the flatness of the surface, the axial furrows are 
clearly defined, and the segmentation of both axis and pleure are discernible even to the extremity of the shield, especially 
on the internal casts. The fossil is not especially common, though I have seen several characteristic examples. Thespecies 
may be distinguished by the term, [sotelus jacobus, being dedicated to Prof, James Hall. Me 
