282 The American Naturalist. [April, 



skeleton, but, in its later forms, is capable of rolling up into a 

 hard ball with no part of its body exposed. Evidently the 

 destroyer himself in time came into peril and needed protec- 

 tion. Some still more powerful and voracious foe had come 

 upon the field, and the triumphant trilobite was forced to 

 acknowledge defeat. 



We cannot well imagine any of these animals assuming such 

 armor except for protective purposes. The weight laid upon 

 them rendered them slow and sluggish, fixed some of them 

 immovably, and greatly decreased their powers of foraging. 

 The only cause which seems sufficient for their assuming this 

 disadvantageous condition is that of imminent peril— a peril 

 which affected all known forms alike. 



Whence came this peril ? Where is the voracious foe against 

 whom they all put on armor, even the preceding master of the 

 seas? No trace of such a creature has been found. In truth, 

 we cannot fairly expect to find it, since it was probably desti- 

 tute of hard parts, and left behind it nothing to be fossilized. 

 It had no foe and needed no armor, while lightness and flexi- 

 bility may have been of such advantage to it that armor would 

 have proved a hindrance. It probably was a swimming creat- 

 ure and thus left no impress of its form upon the mud. It is 

 to this unknown creature that we must ascribe the armored 

 condition of all known forms of life at that period, even the 

 later cephalopods, large and powerful mollusks, becoming 

 clothed in a cumbrous defensive shell, which they were obliged 

 to drag about with them wherever they went. 



It is a strange state of affairs which thus unfolds before our 

 eyes. All the life we know of seems diligentlv arming itself 

 against some terrible enemy, which itself has utterly vanished 

 and left as the only evidence of its existence this display of 

 universal dread. The creature in question would appear to 

 have been without internal or external hard skeleton and with- 

 out teeth, trusting to indurated jaws for mastication. At a later 

 date, when its prey became less easily destroyed, teeth may 

 have developed, and it is possible that we have remains of 

 them in the hard, cone-like, minute substances found in the 

 lower Silurian strata, and known as conodonts 



