SUMMARY. 147 



dence is far from clear, it would appear that the discovery of animals with the form of 

 Limiting 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 Aglaspidae, as represented by Malaria and Habclia 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 antenme, the absence of facial sutures, and the possession of a spine-like 

 telson, 



The Aglaspidnc 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-\\\x, 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 Aglaspidae the progenitors of the limulids. 



The oldest known Lintnliis-like animal other than the Aglaspidae is Ncolimulns falcatus 

 Woodward (Geol. Mag., dec. i, vol. 5, 1868, p. i, pi. i, fig. i). The structure of the head of 

 this animal is 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. Bclinurus, from the Mississippian 

 and Pennsylvania!!, has a sort of pygidium, the posterior three segments being fused together, 

 and Prcstwichia 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 Neolimulus to the Belinuridie of the late Palaeozoic and the Limu- 

 lidre 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 paheontologic 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 



