258 S. W. Ford—Additional Embryonic Forms of Trilobites. 
the evolution of some Paradoxides-like form, then the line ot 
descent probably extends backward through the macropleural 
section of the genus Olenelius to some such species as Para- 
doxides Kjerulfi, or perhaps to some still more divergent form 
of Paradoxides, with which we are as yet unacquainted. 
uh 
( 
y) 
Fig. 12.—Adult specimen of Olenellus Thompsoni Hall, reduced one-half. Fig. 
on. Medium sized individual of O. Vermontanus Hall, natural size. Both after 
all. 
Hydrocephalus is a still somewhat obscure genus occurring in 
the Bohemian Primordial; but, as long since pointed out by 
Barrande, one of the close allies of Paradoxides. It differs 
from Paradoxides mainly in the course of its facial sutures, and 
in the peculiar position of its genal spines; the former striking 
the posterior margin, according to Barrande, in such a way as 
to leave the latter attached to the fixed cheeks (see fig. 11, a @). 
Barrande considers the head to have had the form shown in 
