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 Pcedewnias, 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 

 (Pccdcumias*) 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 Dalmanitcs 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- 

 copidae, Proetidze, Calymeniche, 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 Trinucleidze, 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 

 Bathyuridse. The absence of compound eyes and the poor development of ocelli in the Har- 

 pedidas suggest that they were burrowers, and from these two families, Trinucleidse and 

 Harpedidae, 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 



