TRILOBITA. 



361 



ascribes these to the Limuloid genus Belinurus. The curious track 

 known as Climactichnites (fig. 217, 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 Protichnites, and 

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

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

 believes that both Protichnites and Climactichnites are really of vegetable 

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



A B 



Fig. 217. Supposed Crustacean tracks and burrows. A, Portion of the track of Protich- 

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

 B, Portion of the track of Protichnites lineatus, 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) Mlobatus, from the Clinton formation (Middle 

 Silurian), reduced one-half. (After Owen and Hall.) 



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

 the Silurian 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 burrows of Trilobites, and that 

 it can be shown to be sometimes connected with footprints consisting of 

 a double series of transverse markings. In form, Rusichnites (fig. 217, D) 

 presents itself as an oval, cylindroidal body, deeply furrowed or bilobed, 

 by means of a longitudinal sulcus, the lateral halves being transversely 

 ridged or grooved. The body thus constituted may be independent, 

 or may stand in apparent connection with a cylindrical and slender 

 appendage. 



As regards their zoological affinities, the Trilobites are 

 related most closely to the Phyllopoda and the Xiphosura, 

 though they also exhibit points of agreement with the Iso- 



