100 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Dollo (1910, p. 406), and quickly following his lead, Staff and Reck (1911, p. 130), have 

 published extremely suggestive papers, showing that by the use of the principle of correlation 

 of parts, much can be inferred about the mode of life of the trilobites merely from the 

 structure of the test. 



Dollo studied the connection between the shape of the pygidium and the position and 

 character of the eyes. As applied by him, and later by Clarke and Ruedemann, to the euryp- 

 terids, this method seems most satisfactory. He pointed out that in Eurypterida like Sty- 

 lonurus and Eurypterus, where there is a long spine-like telson, the eyes are back from the 

 margin, so that a Limulns-l\ke habit of pushing the head into the sand by means of the limbs 

 and telson was possible. Ercttopterus and Pterygotus, on the other hand, have the eyes on 

 the margin, so that the head could not be pushed into the mud without damage, and have 

 a fin-like telson, suggesting a swimming mode of life. 



In carrying this principle over to the trilobites, Dollo was quite successful, but Staff 

 and Reck have pointed out some modifications of his results. The conclusions reached 

 in both these papers are suggestive rather than final, for not all possible factors have been 

 considered. The following are given as examples of interesting speculations along this line. 



Homalonotus dclphinoccphalus, according to Dollo, was a crawling animal adapted to 

 benthonic life in the euphotic region, and an occasional burrower in mud. This is shown by 

 well developed eyes in the middle of the cephalon, a pointed pygidium, and the plow-like 

 profile of the head. This was as far as Dollo went. From the very broad axial lobe of 

 Homalonotus it is fair to infer that, like Isotclus, it had very long, strong coxopodites which 

 it probably vised in locomotion, and also very well-developed longitudinal muscles, to be used 

 in swimming. From the phylogeny of the group, it is known that the oldest homalonotids 

 had broad unpointed pygidia of the swimming type, and that the later species of the genus 

 (Devonian) are almost all found in sandstone and shale, and all have wider axial lobes 

 than the Ordovician forms. It is also known that the epistoma is narrower and more 

 firmly fused into the doublure in later than in earlier species. These lines of evidence tend 

 to confirm Dollo's conclusion, but also indicate that the animals retained the ability to swim 

 well. 



On the same grounds, Olenellus thompsoni and Dalmanites limulurus were assigned the 

 same habitat and habits. Both were considered to have used the terminal spine as does 

 Limulus. 



Olenellus thompsoni is generally considered to be unique among trilobites in having a 

 Limulus-like telson in place of a pygidium. This "telson" has exactly the position and 

 characteristics of the spine on the fifteenth segment of Mesonacis, and so long ago as 1896, 

 Marr (Brit. Assoc. Adv. Sci., Rept. 66th Meeting, page 764) wrote : 



The posterior segments of the remarkable trilobite Mesonacis vermontana are of- a much more delicate 

 character than the anterior ones, and the resemblance of the spine on the fifteenth "body segment" of this 

 species to the terminal spine of Olenellus proper, suggests that in the latter subgenus posterior segments of a 

 purely membranous character may have existed devoid of hard parts. 



This prophecy was fulfilled by the discovery of the specimens which Walcott described 

 as Pcedeumias transitans, a species which is said by its author to be a "form otherwise 

 identical with O. thompsoni, [but] has rudimentary thoracic segments and a Hohnia-like 

 pygidium posterior to the fifteenth spine-bearing segment of the thorax." A good speci- 

 men of this form was found at Georgia, Vermont, associated with the ordinary specimens 

 of Olenellus thompsoni, and I believe that it is merely a complete specimen of that species. 



