78 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery Gallery. With these latter are other obscure fossils, named 

 Salterella and Arenicolites from the Basal Quartzites of Lower 



Table-case Cambrian age in Sutherland. All these are supposed to 

 2®- have been worm-burrows like those of the living lob-worm 

 Arenicola, but they may equally well have been made by 

 plant-roots. Some supposed tubes of Serpulites from the 

 same Quartzites have a slightly better claim to an annelid 



Wall-case origin. On the top shelf of the Wall-case is another Areni- 

 colites from the Upper Cambrian of Wisconsin ; in this the 

 burrows are seen to stop short at the level of successive 

 layers of rock, as though the animals had been killed off, 

 either by a period of drought or by the sudden deposition of 



h c 



Fig. 35. — The tubicolous polychsete Ortonia. a, Ortonia intermedia, 

 from the Devonian of Canada : h, 0. conica, adhering to a brachiopod 

 shell, from the Ordovician (natural size) ; c, a single tube of the same, 

 enlarged (after Nicholson). 



a thick coating of sand. A similar form called Scolitlius 

 comes from the Potsdam Sandstone of the same age near 

 Ottawa; the bit of rock exhibited on the bottom slope of 

 the Wall-case shows over 80 burrows on a surface no bigger 

 than a man's hand. 

 Wall -case Among British Silurian specimens may be noted the 

 large Serpulites longissimus, of which a tube curved in an 

 almost complete circle is at the back of the Wall-case, the 

 Table-case small coiled Spirorhis (Fig. 36), and the ringed tubes of 

 Wan^ease ^^^^'^^^'^^^^ often found in clusters. The similar tubes of 

 15a. Ortonia are attached to shells and such-like objects (Fig. 35). 

 From the Lower Devonian of Cornwall come some peculiar 



