32 PALEONTOLOGY OF THE EUREKA DISTRICT. 
OBSERVATIONS ON OLENELLUS HOWELLI. 
Plate xxi, figs. 1-17. 
The study of the material in the collection illustrating this species hay- 
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 series 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, tig. 
1, is 2.5"" in length, and resembles in its outline the larger head, fig. 3, 
which has a length of 7.5™", as is seen more clearly by comparing the latter 
with the enlarged figure of fig. 1, on plate ix, fig. 150. 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 22, 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 
