102 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
and spinose. That such a method of life was secondary in a cheirurid was indicated to 
him by the fact that the more primitive members of the family seemed adapted for crawl- 
ing. Staff and Reck have gone further and shown that the pygidium is only the vestige of 
a swimming pygidium, and that the great development of spines suggests a floating rather 
than a swimming mode of life. They therefore argue for a planktonic habitat. A similar 
explanation is suggested for Acidaspis and other highly spinose species. 
The Aeglinidae, or Cyclopygidse as they are more properly called, present the most re- 
markable development of eyes among the trilobites. In this, Dollo saw, as indeed earlier 
writers have, an adaptation to a region of scanty light. The cephalon is not at all adapted 
to burrowing, but the pygidium is a good swimming organ, and one is apt to agree that this 
animal was normally an inhabitant of the ill lighted dysphoric region, but also a nocturnal 
prowler, making trips to the surface at night. Similar habits and habitat are certainly indi- 
cated for Telephus and the Remopleuridas, but whether Nileus and the large-eyed Bumastus 
are capable of the same explanation is doubtful. 
Finch (1904, p. 181) makes the suggestion that "Nileus" {Vogdesia) vigilans, an 
abundant trilobite in the calcareous shale of the Maquoketa, was in the habit of burying itself, 
posterior end first. He found a slab containing fifteen entire specimens, all of which had 
the cephalon extended horizontally near the surface of the stratum, and the thorax and 
pygidium projecting downward. The rock showed no evidence that they were in burrows, 
and the fact that all were in the same position indicates that the attitude was voluntarily 
assumed. They appear to have entrenched themselves by the use of the pygidia, which are 
incurved plates readily adapted for such use, and, buried up to the eyes, awaited the coming 
of prey, but were, apparently, smothered by a sudden influx of mud. The form of the eye 
in Vogdesia vigilans bears out this supposition of Finch's. Not only are the eyes unusually 
tall, but the palpebral lobe is much reduced, so that many of the lenses look upward and 
inward, -as well as outward, forward and backward. The particular food required by V. vigi- 
lans must have been very plentiful in the Maquoketa seas of Illinois and Iowa, for the species 
was very abundant, but that its habits were self -destructive is also shown by the great num- 
ber of complete enrolled specimens of all ages now found there. The soft mud was appar- 
ently fatal to the species before the end of the Maquoketa, for specimens are seen but very 
rarely in the higher beds. 
Vogdesia vigilans is shaped much like Bumastus, Illcenus, Asaphus, Onchometopus, and 
Brachyaspis, and it may be that these trilobites with incurved pygidia had all adopted the 
habit of digging in backward. As noted above, their pygidia are not very well adapted 
for swimming, and most of them have large or tall eyes. 
Dollo's comparison of the Cyclopygidse to the huge-eyed modern amphipod Cystosoma 
is instructive. This latter crustacean, which has the greater part of the dorsal surface of the . 
carapace transformed into eyes, is said to live in the dysphoric zone, at depths of from 40 
to 100 fathoms, and to come to the surface at night. It swims ventral side down. 
The kinds of sediments in which trilobites are entombed have so far afforded little evi- 
dence as to their habitat. Freeh (Lethaea palseozoica, 1897-1902, p. 67 et seq.) who has 
collected such evidence as is available on this subject, places as deeper water Ordovician 
deposits the "Trinucleus-Schiefer" of the upper Ordovician of northern Europe and Bohemia, 
the "Triarthrus-Schiefer" of America, the "Asaphus-Schiefer" of Scandinavia, Bohemia, 
Portugal, and France, and the Dalmania quartzite of Bohemia. 
Cryptolithus and Triarthrus, although not confined to such deposits, are apt to occur 
