706 



THE PALEONTOLOGY OF MINNESOTA. 



[Isotelus glgas. 



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 Isotelus, 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. 



ISOTELUS GIGAS 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.f 



Fig. 8. Isotelut gigas DeKay. Hypostoma of a large individual, 

 (erratic), central New York. 



Hudson River sandy shales 



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, 

 which 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. 



ISOTELUS MAXIMUS Locke, 1838. 



This is the name first proposed by Locke, who subsequently changed it, for 

 euphony, to /. megisfos, under which it has usually passed. 



Formation and locality. Trenton: Minneapolis, St. Paul, Rochester, Minn.; Mineral Point, Wis. 

 Galena: Wykoff, Warsaw, Konyon, Cannon Falls, Minn. Hudson River: Granger, Minn. 



Trans. Geologlr.-il S.K-ii-iy. vol 1.2nd Series, pp. 199,208, pi. 27. 1822. 



+Thi- Tri'iiinn riH-ksnf New York contain adistinct species known at present only from Its pygldlum. Thishas passed 

 under the n:iuir /. f/ly<r. and Is (loured In the Palaeontology of New York, vol. I, I pi. 61, figs 30 3d . associated with ccpbala 

 of correspond 1 111: -.!/. (nil which may or muy not lielonK to It. It I- < -li:ii -aetei i/i -i\ by Its brond, blunt, somewhat elevated 

 posterior termination, and flat upper surface and axis. Notwithstanding Hie flatness of the surface, the axial furrows are 

 clearly defined, and the segmentation of both axis and pleura' are discernible even to the extremity of the shield, especially 

 on the Internal casts. The fossil i, mil . -]> rialh common, though I have seen several characteristic examples. Thespeciea 

 mmy be distinguished 1>% tin term. Imitrlux Jn< "//>/.-. ln-im; ili'ilirated to Prof. James Hall. 



