TRILOBITA. 



525 



ProtichnUes. The tracks upon which this genus is founded (fig. 381, A 

 and b) consist essentially of a median groove, with a number of depres- 

 sions or footprints on each side in corresponding pairs, these being often 

 arranged in answering groups, of seven or eight pairs each. Sometimes 

 the pits or footprints are replaced by shallow grooves, on each side of the 

 main median groove (fig. 381, B). The tracks of ProtichnUes indicate 

 the existence in the Upper Cambrian of some animal of very considerable 

 size, since they are sometimes half a foot or more in width. That the 

 animal producing these tracks was a Crustacean can hardly be doubted ; 

 the median groove being made by the tail-spine, and the lateral mark- 

 ings by the feet ; and as we know that large Trilobites actually lived 

 during this period, it seems most reasonable to suppose that we have in 

 these the real makers of the tracks. Sir William Dawson, however, has 

 shown that tracks of a closely similar nature are formed by the living 



'~\\ 



F~~3 fit A 



I I ■ ; : 





A 



D 



Fig. 38T. — Supposed Crustacean tracks and burrows, a, Portion of the track of Protich- 

 nites altertians, from the Potsdam Sandstone, reduced to one-tenth of the natural size; b, 

 Portion of the track of ProtichnUes lineattis, from the same formation, similarly reduced ; c, 

 Portion of Climactichnites Wilsoni, from the Potsdam group, reduced to one-thirtieth of the 

 natural size ; d, Rusichnites {Rusophycus) bilobatus, from the Clinton formation (Silurian), re- 

 duced one-half. (After Owen and Hall.) 



King - crabs (Limuius), and he would therefore ascribe ProtichnUes 

 rather to the Eurypterids. The same eminent observer has also shown 

 that smaller forms of ProtichnUes occur in the Carboniferous ; and he 

 ascribes these to the Limuloid genus Belinnrus. The curious track 

 known as Climacticlmites (fig. 381, c) is likewise found in the Potsdam 

 formation, and consists of a band about six inches wide, crossed by 

 straight ridges, and bounded by beaded margins. These were probably 

 formed by the same animal as that which produced ProtichnUes, and 

 Dawson has shown that the living Linudus, when it uses its swimming- 

 feet, gives rise to a ladder-like track of the same kind. Prof. Chapman 

 believes that both ProtichnUes and Climactichnites are really of vegetable 

 origin. The only other fossil which need be mentioned in this connec- 

 tion is the curious Rusichnites, which is of common occurrence in the 

 Ordovician and Silurian rocks of North America, and is also found 

 in the Carboniferous. Originally described as a plant under the name 

 of Rusophycus, its name was changed by Dawson to Rusichnites, in 

 accordance with his belief that it really represents the casts of the bur- 



