SUMMARY. 14/ 
dence is far from clear, it would appear that the discovery of animals with the form of 
Limuhis and the eurypterids and the appendages of trilobites means something more than 
descent from similar ancestors. Biramous limbs of the type found in the trilobites would 
probably not be evolved independently on two lines, even if the ancestral stocks were of 
the same blood. 
The Aglaspidse, as represented by Molaria and Habelia in the Middle Cambrian, are 
quite obvious closely related to the trilobites, easily derived from them, and retain numer- 
ous of their characteristics. That they are not trilobites is, however, shown by the presence 
of two pairs of antenna?, the absence of facial sutures, and the possession of a spine-like 
telson. 
The Aglaspidse have always been placed in the Merostomata, and nearer the Limulidse 
than the Eurypterida. The discovery of appendages does not at all tend to strengthen that 
view, but indicates rather that they are true Crustacea which have not given rise to any 
group now known. The exterior form is, however, Limulus-like, and since it is known from 
ontogeny that the ancestor of that genus was an animal with free body segments, there 
is still a temptation to try to see in the Aglaspidse the progenitors of the limulids. 
The oldest known Lit •nidus -like animal other than the Aglaspidae is Ncolimulus falcatus 
Woodward (Geol. Mag., dec. 1, vol. 5, 1868, p. 1, pi. 1, fig. 1). The structure of the head of 
this animalis typically limuloid, with simple and compound eyes and even the ophthalmic 
ridges. Yet, curiously enough, it shows what in a trilobite would be considered the posterior 
half of the facial suture, running from the eye to the genal angle. The body is composed 
of eight free segments with the posterior end missing. Bclinnrus, from the Mississippian 
and Pennsylvanian, has a sort of pygidium, the posterior three segments being fused together, 
and Prestwichia of the Pennsylvanian has all the segments of the abdomen fused together. 
So far as form goes, a very good series of stages can be selected, from the Aglaspidse of 
the Cambrian through Neolimuhis to the Belinuridae of the late Palaeozoic and the Limu- 
lidae of the Mesozoic to recent. Without much more knowledge of the appendages than is 
now available, it would be quite impossible to defend such a line. It is, however, sug- 
gestive. 
EVOLUTION OF THE "TRACHEATA." 
The trilobites were such abundant and highly variable animals, adapting themselves to 
various methods of life in the sea, that it appears highly probably that some of them may 
have become adapted to life on the land. The ancestors of the Chilopoda, Diplopoda, and 
Insecta appear to have been air-breathing animals as early as the Cambrian, or at latest, 
the Ordovician. Since absolutely nothing is yet known of the land or even of the fresh- 
water life of those periods, nothing can now be proved. 
In discussing the relationship of the trilobites to the various tracheate animals, I have 
pointed out such pakeontologic evidence as I have been able to gather. Studies in the field 
of comparative morphology do not fall within my province. I only hope to have made the 
structure of the trilobite a little more accessible to the student of phylogenies. 
SUMMARY ON LINES OF DESCENT. 
In order to put into graphic and concise form the suggestions made above, it is neces- 
sary to define and give names to some of the groups outlined. The hypothetical ancestor 
