318 



ANNULOSA. 



Scolithus, Histioderma, and Arenicolites, all of which occur 

 in rocks of Cambrian or Silurian age. Scolithus is founded 

 upon long burrows, which are nearly straight, and descend 

 vertically through the rock (fig. 189). They often become 

 somewhat widened out superiorly, and are generally found 

 in great numbers together. They occur abundantly in the 

 Potsdam Sandstone (Upper Cambrian), and Clinton forma- 

 tion (Upper Silurian) of North America, and also in the 

 hard sandstones of the Stiper Stones in Shropshire (Upper 

 Cambrian). They have been supposed to have been formed 



Fig. 189. Annelide-burrows (Scolithus Canadensis), from the Potsdam Sandstone (Upper 

 Cambrian). (After Billings.) 



by sea-weeds, but there is little doubt that they are truly 

 the burrows of Annelides. The somewhat problematical 

 fossil upon which the genus Histioderma is founded is de- 

 scribed as a curved burrow, from one to 

 nearly four inches in length, terminating in 

 a trumpet-shaped opening, which is placed 

 in the centre of a small mound. The genus 

 Arenwolites, again (fig. 190), includes small 

 rig. i9o. Bun-owe double burrows, which form loops, shaped 

 utton the Long! like tn letter U, opening on the surface 

 (L wer Cam " ky two apertures placed close to one another. 

 The mouths of these burrows are thus placed 

 in pairs, one orifice being supposed to be an aperture of 

 entrance for the worm, and the other one of exit. Burrows 



