ANNELIDA. 



311 



the order, though many of the fossils referred to this group 

 are of a more or less problematical nature. The genus 

 Spirorbis has survived from the Upper Silurian period to the 

 present day ; and forms very nearly allied to, if not actually 

 identical with, the recent Serpulce, are found in almost all 

 formations, beginning with the Silurian. 



The chief Palaeozoic genera of Tubicola are Cornulites, 

 Conchicolites, Ortonia, Serpulites, Tr achy derma, Spirorbis (Mic- 

 roconchus), and Serpula. The 

 genus Cornulites (fig. 181) 

 is Silurian, and the best 

 known species is C. scrpu- 

 larius. In this singular 

 form the tube is of consider- 

 able length often three or 

 four inches with a wide 

 aperture at one end, and 

 tapering gradually to its 

 opposite extremity, which 

 is often curved, and seems 

 to have been attached to 

 some solid body. The tube 

 is calcareous, with very 

 thick walls, the substance 



of which is composed of a number of cellular cavities. Exter- 

 nally the tube is ringed with transverse annulations, and 

 marked with fine longitudinal striae. The internal cast of 

 the tube has the form of a series of inverted conical rings, of 

 small width, arranged in an imbricated manner. The tube 

 appears to have been solitary, and is rarely found attached. 

 The microscopic structure of the thick wall of Cornulites is 

 very peculiar, and exhibits a thin outer laminated layer 

 and a delicate internal wall, the space between these being 

 divided into small lenticular compartments or cells by means 

 of curved calcareous septa. So peculiar is this structure, 

 that it raises some doubt as to the true nature and affinities 

 of Cornulites. 



In the genus Conchicolites (fig. 182) we have a much 

 smaller Annelide, living socially, and forming masses of 



Fig. 181. Cornulites serpular^^ls. Upper 

 Silurian. The right-hand figure represents a 

 cast of the tube. (Original.) 



