Order 19. =Squamariaceæ.= The Algæ belonging to this order
form crust-like coverings on stones, mussel-shells, and on
other Algæ, but are not themselves incrustated: _Petrocelis_,
_Cruoria_, _Peyssonellia_.
Order 20. =Corallinaceæ.= Partly crustaceous, partly erect,
branched Algæ, thickly incrusted with lime, so that a few
species (_Lithothamnia_, also called _Nullipora_) occur in
fossilized condition from Jurassic to Tertiary periods.
_Melobesia, Lithophyllum, Lithothamnion, Corallina._
USES. “Carragen” is the thallus of _Chondrus crispus_ (Irish Moss)
and _Gigartina mamillosa_. It is a common article of food on the
coasts of Ireland, and swells to a jelly when cooked. It is officinal.
_Rhodymenia palmata_ is generally eaten as food in Ireland and in some
places on the west coast of Norway; it is also used as food for sheep
and hence is termed “Sheep-seaweed.” Agar-Agar is the jelly obtained
from species of _Gelidium_ and _Gigartina_ growing in China and Japan.