PLATE XXI. 



Page. 

 OLENBLLUS HOWELLI 30 



Fig. 1. Outline of the smallest head of this species discovered. Natural size. 



2. Head showing the anterior position of the genal spines, g g, and the angles of the 



posterior margin, xx, extravagantly developed. The difference in the length of 

 the ocular ridges of the right and left side is also very marked. Natural size. 



3. The smallest individual in which the posterior course of the facial suture was 



observed. The outline of the head is much like that of fig. 1. Natural size. 



4. Form intermediate in contour of head, between figs. 2 and 3. g g, Genal angles 



and spines; xx, angles of the posterior margin. Natural size. 



6. The eyes in this specimen are no longer peduuculated or united to the glabella by 

 an ocular ridge, and the genal angles are more posterior. The course of the 

 facial suture in front of the eye is also seen for the firt time. Natural size. 



6. Example in which the genal angles are in the same position as in the adult indi- 



vidual in species of the genus. The eyes are more embryonic in character than 

 in the preceding example. Natural size. 



7. Narrow form, with the eyes of the adult type, and having the genal angles carried 



forward, as in the younger individuals, 1, 3, 5. Natural size. 



8. Broader and more common form showing the same peculiarities as fig. 7. Natural 



size. 



9. The right and left sides are irregularly developed, the genal spine on the left side 



being more anterior in position. The course of the facial suture is traced in 

 accordance with its position, as observed in fig. 6. Natural size. 



OLENBLLUS ASAPHOIDES Emmons 36 



Fig. 10. Embryonic form, showing the circular outline, the genal spines in close proximity 

 to the facial suture, and the interocular spines. Enlarged to 3.5 diameters. 

 (After Ford.) 



11. Another phase of the development of this species, succeeding, with probably in- 



termediate forms, fig. 10. The posterior of the genal spines, g g, and the sutures 

 cutting the posterior margin at the angles, x x, are comparable to the same in fig. 3. 

 Enlarged to five diameters. (After Ford.) 



12. Normal adult type of*head of this species, enlarged to two diameters. (After Ford.) 



OLENELLUS GILBERT: -.29 



Fig. 13. Small head, natural size, for comparison with figs. 2, 3, etc., of its associate species, 



0. Howdli, A large head is figured on plate ix, fig. 16. 



14. Narrow form of head, that shows the angles in the posterior margin, xx, slightly 

 developed. Natural size. (After White.) 



PARADOXIDES KJERULFI Linnarsson 39 



Fig. 15. Outline of head showing the position of the genal angles and angles of the pos- 

 terior margin, xx, with the interocular spine; also the ocular ridge (c) uniting 

 the glabella and eyes. (After Linnarsson.) 



OLBNELLUS HOWELLI 30 



16. Outline of hypostoma found associated with this species. Enlarged to three diam- 



eters. 



17. Free cheek found detached from the fixed chdfek. The direction of the suture in 



front of the eye is well definedf, ; its direction posteriorly may be along the line 

 of fracture, o o, but it is impossible to satisfactorily determine it. If, as in fig. 6, 

 it would followthe dotted line, o x. 



