ANNELIDA. 



139 



Fig. ^.-Conchicolites gregariiis, growing upon 

 the shell of an Orthoceras. Lower Silurian. 



In the genus Conchicolites (fig. 88) we have a much smaller 

 Annelide, living socially, and forming masses of clustered tubes 

 growing attached to dead 

 shells in Lower Silurian 

 strata. As in Cornulites, 

 the tube of Conchicolites ap- 

 pears to have been calcare- 

 ous, but it is comparatively 

 thin, and has none of the 

 vesicular structure so char- 

 acteristic of the former. The 

 tube of Conchicolites is made 

 up of a series of short coni- 

 cal rings, inserted into one 

 another in an imbricated 

 manner, with their broader 

 ends turned away from the 



mouth of the tube. It is worthy of notice that the casts of 

 Conchicolites, from their possession of the above structure, ex- 

 hibit a close resemblance to the shells of the Silurian genus 

 Te?itaculites ; whilst casts of the shells of some species of the 

 latter are absolutely undistinguishable, if fragmentary, from 

 casts of the tubes of the former. This is a remarkable fact, 

 since Tentaculites has often been regarded as a genus of Tubi- 

 colar Annelides ; but there are strong reasons for believing 

 that it is truly referable to the Mollusca, and belongs to the 

 order of the Pteropods. 



The genus Serpulites was instituted by Murchison for certain 

 smooth semi-calcareous tubes, often of great length, and ap- 

 parently 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 disappeared, there can be little doubt 

 entertained as to their Annelidous nature. 



The genus Spirorbis (fig. 89) is characterised by the posses- 

 sion 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 commences to 

 be represented in the Upper Silurian Rocks, in which S. Lewisii 

 is an abundant fossil. Other species occur in the Devonian, 



