66 
AUG. F. FOERSTE 
not be determined with certainty. Anteriorly the facial sutures 
are almost marginal, being only 0.5 nun. distant from the an- 
terior margin of the cephalon. They almost meet the anterior 
margin of the cephalon, at points about 37 nun. on each side of 
its median line. The length of the genal spines is not known, 
but, judging from the width of their proximal ends, the tip of 
these spines must have extended at least as far as the posterior 
margin of the fourth segment of the thorax. 
The length of the thorax (plates XIV and XIV A) along its 
median line is 12.5 cm.; and its width is about 25.3 cm. There 
are eight segments, as in other species of Isotelus in their adult 
state. The width of the axial lobe is 10 cm. The broad median 
groove along the proximal half of the pleural segments, and the 
diagonal ridge crossing their more distal parts, are as in other 
species of Isotelus. 
The posterior end of the pygidium (plate XIV A) is not pre- 
served, but its length is estimated at 13.8 cm., measuring from 
the posterior margin of the thorax; its width is 24.8 cm.; the 
ratio of the length to the width being about 55 per cent. Along 
the antero-lateral angles the surface inclines abruptly down- 
ward, forming a low ridge, posterior to which, proximally, there 
is a broad groove, similar to that along the proximal parts of the 
pleural segments of the thorax. The marginal part of the 
pygidium is inclined downward and is slightly concave, the 
width of this marginal part varying from 3.5 to nearly 4 cm. 
The entire length of the specimen is 36.8 cm., the ratio of this 
length of the entire individual to the width of the pygidium 
being almost equal to the ratio of three to two; and the ratio of 
this length to that of the pygidium alone equals that of twenty- 
seven to ten. From this it is evident that the length of the 
entire individual falls short of equalling three times the length 
of the pygidium. 
Locality and position. The large specimen of Isotelus de- 
scribed above occurred at the western end of the excavation for 
the conduit beneath the Huffman Conservancy dam, six miles 
northeast of the center of Dayton, at an elevation of 745 feet 
above sea, and 162 feet below the base of the Brassfield forma- 
