138 



ANNULOSA. 



organically attached to their tubes, whereas the Molluscs are 

 always attached to their shell by proper muscles. In the fossil 

 condition, however, it may be very difficult to refer a given 

 calcareous tube to its proper place. As a general rule, how- 

 ever, the calcareous tubes of Annelides, such as Serpula, are 

 less regular and symmetrical than those of Vermetus, whilst the 

 latter is partitioned by shelly septa, which do not exist in the 

 former. Again, the tube of Dentalium is open at both ends, 

 whereas it is closed at one extremity in the Serpulce. In the 

 Annelidous genus Ditrupa, however, the tube is open at 

 both ends, so that this distinction is one not universally appli- 

 cable. 



Tubicolar Annelides are known from the Silurian Rocks 

 upwards, almost every great period having representatives of 

 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 Serpula, are found in almost all formations, 

 beginning with the Silurian. 



The chief Palaeozoic genera of Tubicola are Cornulites, 

 Conchicolites, Serpulites, Trachydertna, Spirorbis (Microcon- 



cAus), and Serpula. The 

 genus Cormtlites (fig. 87) 

 is Silurian, and the best 

 known species is C. serpu- 

 larius. In this singular 

 form the tube is of con- 

 siderable length often 

 three or four inches with 

 a wide aperture at one 

 end, and tapering gradu- 

 ally to its opposite extrem- 

 ity, which is often curved, 

 and seems to have been 

 attached to some solid 

 body. The tube is cal- 

 careous, 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. 



Fig. ^.CornMtes serfularius. Upper 

 Silurian. 



