POLYCH.ETA. 475 



ing forms, apparently little different from the fossil ones, abound in 

 recent seas. 



In the genus Ditrupa, the tube is free and unattached, open at 

 both ends, and calcareous in composition, its general aspect being 

 very similar to that of the shell of a Dentalium. The genus appears 

 to have existed in Carboniferous times, but it is not found in any 

 abundance till the Tertiary period is reached. Species of this genus 

 are abundant in the London Clay (Eocene) and in the Red Crag 

 (Pliocene). 



In the Palaeozoic rocks there occur the remains of various or- 

 ganisms which have been commonly referred to the Tubicolous 

 Annelides, but which differ more or less extensively in structure and 

 character from any existing types, their precise nature being thus 

 rendered more or less doubtful. Some of the more important of 

 these may be briefly considered here. Among the most problemat- 

 ical of these are the fossils for which the names of Serpulites and 

 Trachyderma have been proposed. Under the head of Serpulites 

 are included smooth, arcuate, semi-calcareous, glossy, apparently 

 unattached, tubular fossils, which sometimes reach a length of a 

 foot, with a diameter of an inch. In the most typical forms of 

 Serpulites the tube exhibits two small longitudinal ridges or tubular 

 ribs, placed respectively along the convex and concave faces of the 

 fossil ; but these thickened borders do not appear to be constantly 

 present. Forms of Serpulites are found in the Palaeozoic rocks from 

 the Ordovician to the Carboniferous inclusive, but their true affinities 

 are by no means certain. The name of Trachyderma (apparently 

 the same as the Scolecoderma of Salter) was proposed by Phillips for 

 the casts of membranous, flexible tubes which are found in Ordo- 

 vician and Silurian deposits. These are transversely wrinkled or 

 plaited, and as they are usually found to cross the strata obliquely 

 or vertically, it is probable that they are really Annelidous in their 

 nature. 



More remarkable than the preceding is the Ordovician, Silurian, 

 and Devonian genus Cornulites (figs. 339, 340), in which the animal 

 was solitary, and inhabited a conical calcareous tube of considerable 

 size. The tube of Cornulites (fig. 339) is ringed with well-marked 

 transverse annulations, and is usually ornamented with fine longitud- 

 inal striae. The tube gradually tapers towards its lower end, where 

 it appears to have been attached to foreign bodies, the attachment 

 taking place by the pointed and usually bent initial portion of the 

 tube, and not (as in Orto?iia) by the whole of one side. Moreover, 

 many adult examples show no signs of having been attached by the 

 pointed base, so that the tube was probably not invariably fixed to 

 foreign bodies, but may have been free. The tube may be three 

 or four inches long, straight or slightly curved, and with a wide 



