THE LOWER AND UPPER SILURIAN AGES. 73 



succeeded by any similar creutures serving to connect 

 them with modern forms. Hence the most various 

 conjectures as to their nature. They have been sup- 

 posed to be plants, and have been successively re- 

 ferred to most of the great divisions of the lower 

 animals. Most recently they have been regarded by 

 Hall^ Nicholson,* and others, who have studied them 

 most attentively, as zoophytes or hydroids allied to the 

 Sertularise, or tooth- corallines and sea-fir-corallines of 

 our coasts, to the cell-bearing branches of which their 

 fragments bear a very close resemblance. In this 

 case, each of the little cells or teeth at the sides of 

 the fibres must have been the abode of a little polyp, 

 stretchino: out its tentacles into the water, and en- 

 joying a common support and nutrition with the 

 other polyps ranged with it. Still the mode of life 

 of the community of branching stems is uncertain. 

 In some species there is a little radicle or spike at 

 the base of the main stem, which may have been a 

 means of attachment. In others the hollow central 

 disk has been conjectured to have served as a float. 

 Occurring as the specimens do usually in shales and 

 slates, which must have been muddy beds, they could 

 not have been attached to stones or rocks, and the; 

 must have lived in clear water, either seated on the 

 surface of the mud, attached to sea-weeds, or floating 

 freely by means of hollow disks filled with air. After 

 much thought on their structure and mode of oc- 



• See also an able paper by Carruthers, in the Geological 

 Magazine, vol. v., p. 64. 



