TRILOBITES. 715 
Tllznus americanus.] 
space between the side of the axis and the head of the pleure. The width of the space is between one- 
third and one-half the width of the axis. The pleurw are bent at the fulcra at an angle which varies in 
different individuals, from 25° to 45°, and at nearly one-half their length from the side of the axis. 
Pygidium usually a little shorter than the thorax; varying from moderately to rather strongly con- 
vex; the posterior margin broadly and uniformly rounded; the anterior angie truncated nearly half the 
whole length of the pygidium; the straight sides formed by the truncation forming an angle of from 40° to 
60° with the longitudinal axis of the body. The axis of the pygidium is well defined at the anterior 
margin by the dorsal furrows, which die out at about one-third or one-half the length, converging towards 
each other, and sometimes obscurely defining the apex. 
The surface characters of the species are peculiar although somewhat variable. The specimen on 
which the species was originally founded, has the whole of the head and pygidium covered with short 
squamose fissure-like strie; one edge of each fissure being more elevated than the other, gives to the sur- 
face a wrinkled appearance. These strive vary in length from half a line to two or three iines, and are 
from one-eighth to one-fourth of a line distant from each other. On the tail they seem to radiate irregu- 
lariy from the axis asacenter. Near the front margin and parallel with it, are a number of straight 
continuous fissures. This latter character occurs in other species of this genus. In other specimens the 
striz are more distinct and distant, but still are of the same character. In a specimen in Dr. Grant’s 
cabinet, the middle portion of the front of the head is nearly smooth, and in addition to the strix, is 
coarsely punctured. 
Mr. Billings also called attention to the close approach of IL. americanus to I. 
crassicauda Wahlenberg; and I am of the opinion that the same fossil had been 
described by Hall in 1847 (loc. cit.) under the latter name. 
It appears from the observations made by Holm* that this species (J. crassicauda 
Wahl.) has been generally misapprehended from the date of its description (1821). 
Holm has redescribed the specific characters from the type specimens, and, as a 
result, eliminates from this association all other fossils which have been compre- 
hended under this designation. Thus restricted, the author regards the species as 
unknown outside of Dalecarlia, Sweden, and as having a very short vertical range 
“from the youngest layers of the Orthoceras-limestone to the oldest of the Cystidean- 
limestone” (an immediately succeeding zone). With J. crassicauda thus limited, J. 
americanus appears to be its nearest relative; indeed, there is excellent reason for 
holding the latter but a variety of the former. The similarities in the two are both 
general and detailed; the differences which may be indicated are a somewhat deeper 
anterior convexity of the cephalon in the American form, a less sharply limited 
glabella and pygidial axis. Otherwise the contour of the parts, all and several, the 
surface sculpture with its variations on the different parts, marginal outlines and 
curves of facial sutures are all alike. 
Some of the internal casts of the cephalon from the Galena limestone at Wykoff 
show, at the anterior termination of the dorsal furrows, two lunate cicatrices 
like those mentioned in the description of Bumastus trentonensis and B. orbicaudatus, 
In finely preserved specimens from the Trenton of New York there is seen to be a 
‘smooth interruption of the external ornamentation directly over these spots. The 
Minnesota specimens are usually in a dismembered state and are frequently some- 
* Zeitschr. der deutsch. geolog. Gesellsch., vol, xxxii., p. 559, pl. 33, 1880. 
