INVERTEBEATE ANIMALS IN PALEOZOIC EOCKS. 597 



which the central furrow becomes obsolete, and by the genus 

 of Whiteaves, and other forms destitute of a median 



groove *. 



They cannot be sharply divided into genera or species, because of 

 their variability in passing over different kinds of bottom, and of 

 the changes which occur in consequence of the various modes of 

 progression employed by the animals. 



Pig. 1 represents a typical specimen of Busichnites, from my paper 



Pig, 1. — Rusichnites grenvillensis. The cast of a Crustacean 

 Burrow, with part of a trail leading to it, or from it, at a. 

 Cambro-Siluriau ; Grenville, Canada. 



of 1864, and shows traces of the trail leading to or from the cast of 

 the deep burrow or excavation. Pig. 2, Rusichnites acacUcus^ from 

 the Carboniferous, I now regard as a result of successive strokes of a 

 crustacean tail, with marks of the carapace and limbs. The speci- 

 men represented in fig. 3, which is from the Clinton Pormation of 

 Canada (and from the collection of Lieut.-Col. Grant, of Hamilton, 

 Ontario), illustrates the probable origin of these markings, but also 

 suggests the idea of some of them having been the trails or castings 

 of worms rather than marks of crustaceans. It is evident indeed 

 that these markings are closely connected with those named Nereites 

 by Hall, and of which he has figured several kinds from the Clinton 

 formation, ascribing them to Molluscs. Similar objects have been 

 named Psammichnites by Torell, and are supposed by him to resemble 



* Trans. Roy. Soc. Canada, yoI. i. 18S3 ; Section iv. 18S2, pp. 109-111. 



