_Homalonotus delphinocephalus_, 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 _Isotelus_, 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.