Are formed by adhesion

Gideon Algernon Mantell et T. Rupert Jones, The Wonders of Geology or a Familiar Exposition of the Geological Phenomena, Londres, Henry G. Bohn, 1857, p. 277.

Some of the beds contains bitumen ; others are entirely made up of the tubular cases of the caddis-worms, the larva of the caddis-fly (Phryganea), cemented together by calcareo-siliceous matter. These tubes or cases, which are formed by adhesion of small shells, and occasionally of other substances, to the outer surface of the silken case secreted by the insect, are abandonned by the animal when its metamorphosis is completed , and layers of them may often be seen in our ditches and lakes.

The fossil cases (Indusia tubulata) have been cemented together by calcareous infiltration, and form a compact stone, which is employed for building. The attached shells are so minute, that often more than a hundred are affixed to a single case.