142 ANNULOSA. 



matrix of the rock. The most important genera which have 

 been founded upon remains of this kind are Scolithus, Histio- 

 dcrma, and Arenuolites, all of which occur in rocks of Cam- 

 brian age. Scolithus is founded upon long burrows, which are 

 nearly straight, and descend vertically through the rock (fig. 

 92). Histioderma is a somewhat curved burrow, from one to 

 nearly four inches in length, terminating by a trumpet-shaped 

 opening, which is placed in the centre of a small mound. 

 Aretiicolites includes very small burrows which form loops, 

 opening on the surface by two apertures placed close to one 

 another. The mouths of these burrows are thus placed in 

 pairs, one being believed to be an aperture of entrance for the 

 worm, the other of exit. The nature of none of these fossils 



Fig. 92. Annelide-burrows (Scolithus Canademis), from the Potsdam Sandstone (Upper 

 Cambrian). (After Billings.) 



is wholly free from doubt ; but the other genera, which have 

 been regarded as formed by burrowing Annelides, are still 

 more obscure and uncertain. 



As regards the surface-tracks and trails of Errant Annelides, 

 much difference of opinion exists, and the whole subject is 

 shrouded in obscurity. Many of these tracks were regarded 

 by their describers as being actually the body of the worm, as 

 in Crossopodia (fig. 93). Others, as Gordia and Myrianites y 

 are undoubtedly tracks of some animal, but there is no 

 evidence as to their having been produced by Annelides. 

 Others, again, such as Palceochorda, which are also almost cer- 

 tainly the trail of some marine animal, have been described as 

 the remains of plants. A few of these fossils may truly be of 



