Usnea, Peltigera); (b) lecanorine, or scutelliform, which are orbicular and surrounded by a distinct, more or less prominent thalline margin (e.g.
Dimyarian, with orbicular and almost equilateral shell; adherent; hinge without teeth Sand ligament internal.
The form of the leaf shows a very great variety ranging from the narrow linear form with parallel sides, as in grasses or the needle-like leaves of pines and firs to more or less rounded or orbicular - descriptions of these will be found in works on descriptive botany - FIG.
Two genera of Argyopidae (Hyptiotes and Theridiosoma) construct spring-nets out of their incomplete webs of the orbicular type.
Perfect orbicular webs are made by many genera of Argyopidae (Zilla, Meta, Gasteracantha), the best-known example being that of the common garden spider of England, Aranea or Epeira diademata; but these webs are not associated with any tubular retreat except such as are made under an adjoining leaf or in some nook hard by.
Snares of another type consisting of a tangled mass of threads amongst which the spiders pick their way with ease, but which are impassable to insects, are spun by members of the Theridiidae and Pholcidae; but by common consent the so-called orbicular web, so characteristic of the Argyopidae but by no means confined to them, is regarded as manifesting the greatest perfection of instinct in snare-spinning.
On account of its delicacy no web is more difficult to see than one of the orbicular type above described.