48 Field Museum of Natural History — Geology, Vol. IV. 



Isotelus iowensis Owen. Plate XIII, figs. 1-2. 



/. iowensis Owen, 1852. Rept. Geol. Surv. Wis., la. and Minn., p. 

 577, pi. Ila, figs. 1-7. 



"The general form and contour of the cephalic shield closely re- 

 semble that of /. gigas DeKay; but the facial sutures do not converge in 

 front to form a distinct angle, but describe three parts of a circle as in 

 Asaphus expansus. The eyes are reticulated and the middle lobe of the 

 caudal shield is defined (though sometimes somewhat indistinctly), but 

 the segments are only obscurely pronounced. The glabella is but 

 obscurely defined, and the genal angles are produced into spines. The 

 thorax consists of eight segments. 



From /. megistos, it differs in the eyes being set closer together; in 

 the spines being longer, extending as low as the caudal shield; the 

 pygidium more regularly elliptical, and its axial lobe more distinctly 

 defined. 



From the bituminous limestone mouth of Otter Creek, Turkey 

 River, Iowa." 



The original description, of which the above is practically a copy, 

 is so incomplete that it seems advisable to redescribe the species in more 

 detail. 



Body subelliptical, length about twice the greatest breadth, moder- 

 ately convex, trilobation not well developed. Entire surface finely 

 punctate, the punctse being larger and more pronounced on the free 

 cheeks and less conspicuous on the marginal borders than on other 

 parts of the test. 



Cephalon semi-oval in outline, marginal border defined by a mar- 

 ginal furrow which originates on the genal spines, as an angular groove 

 and develops into a shallow concave furrow gradually widening to the 

 front of the glabella. Dorsal furrows shallow, converging toward the 

 median line in passing the palpebral lobes, then diverging to about 

 their original distance apart. Cranidium moderately convex, greatest 

 convexity just in front of the eyes, concave where the marginal furrow 

 crosses it. The anterior margin of the cranidium forms the margin of 

 the cephalon. Fixed cheeks very small ; not well defined. Free cheeks 

 large, with long genal spines, convex near the eyes, not produced in 

 front of the glabella. The facial sutures originate on the posterior 

 margin of the cephalon about midway between the dorsal furrows and 

 the lateral margins, from whence they converge forward in a sigmoid 

 curve to the crest of the eye lobes, which they follow, thence forward 

 and outward in an arcuate curve, meeting the anterior margin of the 

 cephalon in front of the anterior angles of the eyes; here the sutures 

 bifurcate, one fork following the anterior margin until it meets the 



