34 PALEONTOLOGY OF THE EUEEKA DISTRICT. 



increase in size, these features usually disappear, although in fig. 6 there 

 is an exception, as they are retained in a modified degree after other accom- 

 panying embryonic features of the head have disappeared. Between the 

 eye and glabella, when the former is situated well out on the cheek, a small 

 round or oval boss occurs, as shown in figs. 2, 3, 4, and 6. 



Facial suture. The course of the facial suture in front of the adult 

 head is shown by the free cheek, fig. 1 7 (s). Of its variation from this course 

 during the development of the individual nothing is known from actual 

 observation, but from the fact that the relative position of the eye changes, it 

 is probable almost to a certainty, that its direction is somewhat varied, and 

 we know that such is the case in its direction back of the eye when the 

 latter is situated out on the cheek or near the glabella. Back of the eye, 

 in all the instances in which it has been observed, its direction is slightly 

 outward, with a sigmoidal curvature to the angle of the posterior margin, 

 or, in its absence, at the broad curve denoting the position of the angle 

 about midway between the dorsal furrow and the genal angle, figs. 3, 4, 5, 

 and 6. In the large adult specimens its course was probably as indicated 

 by the traced line in fig. 9, where it is much the same as in the genus Ogygia 

 or Dicellocephalus, and even more so in the cheek, fig. 17, if it cut the poste- 

 rior margin at the angle #, as it does in all known cases in the smaller speci- 

 mens, figs. 3, 4, 5, and 6. Comparing this with the direction of the stages of 

 growth shown in figs. 3 and 4, where another generic group is suggested by 

 its Paradoxides-like course, the contrast is very striking. 



In the adult specimens of Olenellus Gilberti and 0. Vermontana the 

 course of the facial suture is almost directly backward from the eye to the 

 margin, corresponding nearly to the line o o, of fig. 17, and terminating far 

 within the angle x. That the course of the suture was the same in the 

 larger heads of 0. Howelli is not probable, as the head, fig. 6, shows it ter- 

 minating at the place of the angle x, But the fact that in the closely allied 

 species 0. Gilberti the termination is between the angle and the dorsal suture 

 would lead to looking for it there in 0. Howelli; that its course, if not con- 

 stant in the large adults, is as in fig. 6, only adds another anomalous feature 

 to this strange species. 



Mode of development. The normal development of a trilobite from the 



