FOSSILS OF THE OAMBEIAN. 



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. 

 15&). 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 o D w 



