ANNELIDA. 



313 



the genus seems to be most abundant in the Lower Silurian 

 (Cincinnati group) of North America, and in the Wenlock 

 Limestone (Upper Silurian) of Britain. 



The genus Serpulites was instituted by Murchison for 

 certain smooth semi-calcareous tubes, often of great length, 



Fig. 183. A, Tubes of Ortonia cornea (Nich.) growing upon the valve of Stroplwmena 

 alternate, natural size ; B, A single tube of the same, enlarged. 



and apparently unattached, which occur in the Silurian series. 

 These tubes in some species reach a length of over a foot, 

 with a diameter of an inch, and their true nature is very 

 doubtful. The genus Trachyderma, again, was proposed by 

 Phillips for the casts of membranous flexible tubes which 

 are found in the Silurian rocks. These are transversely 

 wrinkled or plaited, and though the tube itself has disap- 

 peared, there can be little doubt entertained as to their 

 Annelidous nature. 



The genus Spirorbis (figs. 184-186) is characterised by the 

 possession of a shelly calcareous tube, which is coiled into 

 a flat spiral, one side of which is cemented to some foreign 

 body. The spiral may be either right-handed or left-handed, 

 and the shell generally occurs in numbers together, attached 

 to dead shells or to the remains of plants. The genus comr 

 mences to be represented in the Upper Silurian rocks, in 

 which S. Lewisii is an abundant fossil. Other species occur 

 in the Devonian, often in considerable abundance, attached 

 to the shells of Molluscs or the exterior of corals (figs. 184, 



