168 21ie American Geolucjist. September, 189* 
coarse for the preservation of these small and delicate organ- 
isms. In certain horizons and rocks, however, such remains 
are quite abundant, and complete ontological series may be 
obtained. Yet, it is not strange that series of equal complete- 
ness have not been found in all Paleozoic horizons. 
The abbreviated or accelerated development of many of the 
higher Crustacea has resulted in pushing the typical free- 
swimming, larval nauplius so far forward in the ontogeny 
that this stage is either eliminated or passed through while 
the animal is still within the egg, so that when hatched it is 
much advanced. Although the trilobites show distinct evi- 
dence of accelerated development through the earlier inherit- 
ance of certain characters which will ba taken up later, ye\ it 
is not believed that the normal series or periods of transfor- 
mation were to any degree disturbed, since both the simplest 
and most ])rimitive genera whose ontogeny is known and the 
most highly specialized forms agree in having a common earl}^ 
larval type. This would be expected from their great antiq- 
uity, their comparatively generalized and uniform structure, 
and from the fact that no sessile, attached, parasitic, land, or 
fresh- water species are known. These conditions by introduc- 
ing new elements into the ontogeny would tend to modify or 
abbreviate it in various ways, especially among the higher 
genera. 
Before discussing any of the various philosophical and the- 
oretical problems involved in an attempt to correlate the lar- 
val forms of Crustacea, a brief consideration of the known 
facts relating to the larvae of trilobites will be presented. 
Minute spherical or ovoid fossils associated with trilobites 
have been described as possible trilobite eggs, by Barrande^^ 
and Waleott,*^ but nothing is known, of course, of the embry- 
onic stages of the animals themselves. The smallest and most 
primitive organisms which have been detected, and traced 
by means of series of specimens through successive changes 
into adult trilobites, are, as stated above, little discoid or 
ovate bodies not more than one millimeter in length, as showu 
on plates VIII and IX. It is fair to assume that we have here 
a general exhibition of trilobite larval stages, since the ten spe- 
cies represented are from various geological horizons, belong- 
ing to the Cambrian, Ordovician and Silurian sediments, with 
