32 PALEONTOLOGY OF THE EUEEKA DISTRICT. 



OBSERVATIONS ON OLENELLUS HOWELLI. 



Plate xxi, figs. 1-17. 



The study of the material in the collection illustrating this species hav- 

 ing developed some interesting facts, they are here discussed under two 

 heads: First, the character of the species during the later stages of its 

 development as far as known; second, the relation of the species to other 

 species of the same genus, and to certain species of the genus Paradoxides. 



I. A seiies of specimens illustrating the principal embryonic and abnor- 

 mal features of .development are figured on plate xxi, figs. 1-9, the details 

 of which, and also the figures up to fig. 18, are given in the explanation 

 accompanying the plate. 



Contour of the head. The smallest specimen of the head observed, rig. 

 1, is 2.5 mm in length, and resembles in its outline the larger head, fig. 3, 

 which has a length of 7.5 mm , as is seen more clearly by comparing the latter 

 with the enlarged figure of fig. 1, on plate ix, fig. 156. Between figs. 1 and 

 3, in size, is the form represented by fig. 2, which is transversely quadri- 

 lateral in outline, with the genal angles and spines carried forward to form 

 antero-lateral angles on a line with the frontal margin of the head and the 

 geniculation of the posterior margin, which is so strongly marked in fig 1, 

 is still further increased to form an angle of nearly 90, which, from its 

 position, might be incorrectly viewed as the true genal angle if the anterior 

 spines were broken away or obscured and the course of development of the 

 species unknown. In fig. 4 the genal spines are still more anterior than in 

 the smaller forms (figs. 1 and 3), presenting a transition stage, not consider- 

 ing the size of the head, but the general form, between figs. 1 and 2 or 2 

 and 3, the angles of the posterior margin x x, are also more obtuse and the 

 frontal margin broadly rounded. Through the forms represented by figs. 

 3 to 5 the modification of these features is very uniform to the normal adult 

 type of the contour of the head, as shown by fig. 6. In figs. 7 and 8, how- 

 ever, which are considerably larger specimens, the angularity of the pos- 

 terior margin is a prominent feature, the genal angles being advanced as in 

 the forms before fig. 6. In fig. 9 the outline is still further diversified by 

 having the angularity of the posterior margin and the position of the genal 



