FOSSILS OF THE CAMBRIAN, 33 
spine on one side differing from that on the other. Fig. 6 is considered as 
nearly typical of the adult outline of the head as the largest specimen 
found, and a number of medium size have the same form. The genicula- 
tion of the posterior margin is an embryonic character that is singularly 
persistent in many examples of otherwise adult character. In figs. 12, 13, 
and 14 the adult form of the head of three other species of the genus is 
shown. 
Glabella—There is not as decided a change proportionally between the 
glabella of the smallest individual and that of the normal adult as there is 
in the outer contour of the head; still it is to be observed, that the dorsal 
furrows are not continuous but interrupted by the ocular ridges that cross 
them and unite with the frontal lobe of the glabella (figs. 2, 4, and 6), a feat- 
ure unknown in the adult, that in the younger individuals the glabellar 
furrows extend entirely across the glabella, while in the older ones they are 
more or less interrupted or united only by very shallow transverse furrows 
at the center. 
Eyes.—The usual form of the eye of the adult in this genus is shown 
in figs. 7, 8, 9, 12, and 14, where it is narrow, broadly arching outward, and 
_ with a more or less narrow palpebral lobe close to the glabella. In tracing 
the development of the eye of Olenellus Howelli, it is found, in the earliest 
stage with which we are acquainted, to be situated distant from the glabella 
toward the center of the cheek and united to the anterior lobe of the gla- 
bella by a strong ocular ridge that crosses and interrupts the continuity of 
the dorsal furrow (see fig. 2 and enlargement of fig. 1 on plate ix, fig. 
15b). There is considerable variation in the relative position of the eye 
and the length and strength of the ocular ridge in different examples of 
the head. In fig. 2 the ocular ridge of the left side is elevated, and with 
the high margin of the eyelobe partially incloses the depressed area within, 
while on the right side of the same individual both the ridge and eye are 
less elevated, and the ocular ridge being shorter the eye is brought in 
nearer to the glabella. In fig. 4 the ocular ridge is narrower at the point 
of union with the glabella and attached farther back near the posterior 
margin of the frontal lobe. Frequently the longitudinal axis of the eye is 
slightly oblique from its anterior end backward and outward. With the 
3 CDW 
