LOCOMOTION. IOI 
Olenellus gilberti, which was formerly supposed to have a limuloid telson, has now been 
shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p. 406, pi. 45, fig. 3) to be a 
Mesonacis and to have seven or eight thoracic segments and a small plate-like pygidium 
back of the spine-bearing fifteenth segment. All indications are that the spine was not in 
any sense a pygidium. Walcott states that Olenellus resulted from the resorption of the 
rudimentary segments of forms such as Mesonacis and Pcedeumias, leaving the spine to 
function as a pygidium. This would mean the cutting off of the anus and the posterior 
part of the alimentary canal, and developing a new anal opening on the spine of one of 
the thoracic segments ! 
If the spine of the fifteenth segment is not a pygidium, could it be used, as Dollo 
postulates, as a pushing organ? Presumably not, for though in entire specimens of Olenellus 
(Pcedeumias) it extends back beyond the pygidium, it probably was borne erect, like the 
similar spines in Elliptocephala, and not in the horizontal plane in which it is found in 
crushed specimens. 
While this removes some of the force of Dollo's argument, his conclusion that Olenellus 
was a crawling, burrowing animal living in well lighted shallow waters was very likely cor- 
rect. The long, annelid-like body indicates numerous crawling legs, there is no swimming 
pygidium, and the fusion of the cheeks in the head makes a stiff cephalon well adapted for 
burrowing. 
Staff and Reck have, pointed out that Dalmanites limulurus was not entirely a crawler, 
but, as shown by the large pygidium, a swimmer as well. This kind of trilobite probably 
represents the normal development of the group in Ordovician and later times. The Pha- 
copida;, Proetidae, Calymenidae, and other trilobites of their structure could probably crawl 
or swim equally well, and could escape enemies by darting away or by "digging them- 
selves in." 
Cryptolithus tessellatus (Trinucleus concentricus) is cited by Dollo as an example of 
an adaptation to life in the aphotic benthos, permanently buried in the mud. In this case 
he appealed to Beecher's interpretation of the appendages, and pointed out that while the 
adult is blind, the young have simple eyes and probably passed part of their life in the 
lighted zone. It needs only a glance at the very young to convince one that the embryos 
had swimming habits, so that in this form one sees the adaptation of the individual during 
its history to all modes of life open to a trilobite. The habits of the Harpedidse may have 
been similar to those of the Trinucleida;, but the members of this family are supplied with 
broad flat genal spines. It has been suggested that these served like pontoons, runners, or 
snow-shoes, to enable the animal to progress over soft mud without sinking into it. Some 
such explanation might also be applied to the similar development in the wholly unrelated 
Bathyuridas. The absence of compound eyes and the poor development of ocelli in the Har- 
pedidae suggest that they were burrowers, and from these two families, Trinucleida; and 
Harpedidaa, it becomes evident that a pygidial point or spine is not a necessary part of the 
equipment of a burrowing trilobite. In fact, from the habits of Limulus it is known that 
the appendages are relied upon for digging, and that the telson is a useful but not indis- 
pensable pushing organ. 
Deiphon is an interesting trilobite from many points of view. Its pleural lobes are 
reduced to a series of spines on either side of the body, and its pygidium is a mere spinose 
vestige. Dollo considered this animal a swimmer in the euphotic zone, because its eyes 
are on the anterior margin, its body depressed, its glabella globose, and its pygidium flat 
