256 bulletin: museum of comparative zoology. 



youthful characters usually lost by that species. Thus, the adult 

 Isotelus iowensis has long genal spines, extending to the fifth, sixth, or 

 seventh segment, an axial lobe a little less than half the width of the 

 thorax (.41 to .46), and pygidia show a fairly convex axial lobe, and 

 traces of ribs on the pleural lobes. The pygidia are also somewhat 

 more rounded than those of Isotelus gigas, although fully as long. As 

 minor features of the species, it may be noted that the anterior portion 

 of the glabella is quite well defined, and the geniculation of the pleural 

 lobes of the pygidium is further from the dorsal furrows than in 

 Isotelus gigas. The collection contains a number of cranidia and 

 pygidia, and two complete specimens from the Maquoketa at Elgin, 

 Iowa, and a single complete specimen which is presumably from Iowa 

 but not labeled as to locality, so that direct comparisons can be made 

 with the specimens from Trenton Falls, and the specimens from the 

 eastern and western localities are found to agree in all particulars. 

 The specimens from Trenton Falls have usually been identified as 

 Isotelus maximum. 



Two specimens from the Low\'ille and Black River at Ottawa, 

 Ontario, described by Raymond and Narraway (Ann. Carnegie mus., 

 1910, 7, p. 56, pi. 15, fig. 3) as Isotelus sp. ind. probably belong to this 

 species. They were separated from Isotelus gigas because they had 

 more rounded pygidia with a rather prominent axial lobe, and they 

 have a rather narrow axial lobe in the thorax. In 1910 there was no 

 opportunity of comparing the specimens from Ontario with Isotelus 

 iowensis, but I now find that the^^ agree very closely with the speci- 

 mens from Iowa. 



The specimen from the "Hudson River" at Granger, Minnesota, 

 figured by Dr. Clarke ^ as a specimen of Isotelus maximus, is, as 

 figured, an excellent example of /. iowensis, the shields being too long 

 and narrow for a typical specimen of /. 7uaximus. 



The t^pe of this species, which is now in the collection at the Walker 

 Museum of the University of Chicago, was obtained from the IMaquo- 

 keta shale, which is of Upper Ordovician (Richmond) age, according 

 to the most recent correlations. The specimens from the Trenton, 

 though sometimes larger, are too much like the Iowa specimens to be 

 distinguished as a distinct species at the present time. 



Additional note: — Since this paper was written Mr. A. W. Slocum's 

 excellent account (Field mus. nat. hist., publ. 171, Geol. ser. 4, no. 3, 

 p. 48, pi. 13, fig. 1, 2) of the trilobites of the Maquoketa beds of Fayette 



> PaL Minn., 1897, 3, pt. 2, p. 703, fig. 5; also reproduced by Weller as Isotelus 

 gigas, Pal. N. J., 1902, 3, pi. 14, fig. 5. 



