474 



ANNELIDA. 



name Microconchus has been proposed for forms of Spirorbis, which 

 are not simply cemented by the under surface to some foreign sub- 

 stance, but which make a depression or groove in the body to which 

 the tube is attached. The living species of Spirorbis are all marine, 

 but some of the extinct forms of the Carboniferous period are com- 

 monly found attached to the stems and leaves of undoubted terrestrial 

 plants, a fact which would seem to show that some of these ancient 

 types were able to exist in brackish, or, possibly, even in fresh water. 



Fig. 336. — a, Spirorbis omphalodes, natural size and enlarged — Devonian, Europe and 

 America; />, Spirorbis Arkonensis, of the natural size and enlarged; c, The same, with the 

 tube twisted in the reverse direction — Devonian, America. (Original.) 



Fig. 337. — a, b, Spirorbis iaxus, enlarged — Silurian, America; c, Spirorbis spinidiferus, of the 

 natural size and enlarged — Devonian, Canada. (After Hall and the Author.) 



Fig. 338. — Spirorbis {Microconchus) carbonarius, natural size, attached to a fossil plant, 

 and magnified — Carboniferous. (After Dawson.) 



The earliest known species of Spirorbis occur in the Ordovician 

 rocks, and the genus is abundantly represented in the Silurian, 

 Devonian, an.d Carboniferous rocks, the tubes often occurring in 

 great numbers attached to the exterior of shells or corals. The 

 Spirorbis helicteres of the Carboniferous rocks sometimes occurs in 

 the Carboniferous rocks of Scotland in quantities sufficient to make 

 up bands of limestone of some thickness. Other species have been 

 described from the Permian rocks, and the genus continues to be 

 well represented in both Mesozoic and Tertiary deposits, while liv- 



