GELATINOUS, CIIUSTACEOUS, AND STONE-LIKE ALG^E. 83 
their former consistency, even after they have been laid by 
for a considerable time. 
Those species of Nostoc which are of a coarse dense 
consistency, becoming corrugated when dry, may be 
covered with stearine paper and placed under a light 
pressure. 
Many of the Alga* again are furnished with crustaceous 
or foliaceous fronds, firmly fixed to some underlying sub- 
stance. Of this description are most of the Scytonemacese, 
the genera Hydrococcus, Schizosiphon, Chroolepus, Proto- 
derma, Chlorotylium, Coleochaite, Gongrosira, Hilden- 
brandtia, Melobesia., and several others. Some of these (as 
the Scytonemacea* and Chroolepus) form compact felt-like 
layers on wet shady rocks and walls, or on the bark of 
trees. Others (as Hildenbrandtia) are found encrusting 
stones and pebbles, spreading irregularly over the surface, 
and have a wrinkled leathery appearance. Finally, Hydro- 
coccus, Chlorotylium, and some others, form tiny sponge- 
like excrescences, which at length unite into a mass of rough 
uneven knobs. Some of the smaller kinds (Coleochset *, 
Melobesia, &c.) are parasitic on the larger Algas, and such 
water-plants as Sedges, Mosses, and Potamogeton, where 
they often flourish in the greatest abundance, though 
frequently overlooked, as many of them— the species of 
Coleochaite, for instance— are too minute to be easily detected 
by the naked eye. 
As it is always desirable to secure specimens that give 
a clear conception of their mode of growth, the plants 
should not be rubbed or scraped off, but gathered with 
the object on which they are growing. Where the species 
are at all abundant, small stones are sure to be found 
clothed with the membranous crust peculiar to these Algai ; 
these may be taken and put away just as they are. In the 
case of such as occur upon trees (the Chroolepi, for 
instance), they may be obtained without injury by cutting 
off a portion of the bark. They are more difficult to get 
at when growing on the face of a rock or large block of 
g 2 
