[Illustration: Fig. 39. _Lecanora parella_ Ach. Determinate thallus with white bordering hypothallus, reduced (M. P., _Photo._).] _bb._ =Formation of crustaceous tissues.= Some crustaceous lichens have a persistently scanty furfuraceous crust, the vegetative development never advancing much beyond the first rather loose association of gonidia and hyphae; but in those in which a distinct crust or granules are formed, three different strata of tissue are discernible: 1st. An upper cortical tissue of interlaced hyphae with frequent septation and with swollen gelatinous walls, closely compacted and with the lumen of the cells almost obliterated, not unfrequently a layer of mucilage serving as an outer cuticle. This type of cortex has been called by Hue[322] “decomposed.” It is subject to constant surface weathering, thin layers being continually peeled off, but it is as continually being renewed endogenously by the upward growth of hyphae from the active gonidial zone. Exceptions to this type of cortex in crustaceous lichens are found in some _Pertusariae_ where a secondary plectenchymatous cortex is formed, and in _Dirina_ where it is fastigiate[323] as in _Roccella_. 2nd. The gonidial zone—a somewhat irregular layer of algae and hyphae below the cortex—which varies in thickness according to the species. 3rd. The medullary tissue of somewhat loosely intermingled branching hyphae, with generally rather swollen walls and narrow lumen. It rests directly on the substratum and follows every inequality and crack so closely, even where it does not penetrate, that the thallus cannot be detached without breaking it away. In _Verrucaria mucosa_, a smooth brown maritime lichen found on rocks between tide-levels, the thallus is composed of tightly packed vertical rows of hyphae, slender, rather thin-walled, and divided into short cells. The gonidia are chiefly massed towards the upper surface, but they also occur in vertical rows in the medulla. One or two of the upper cells are brown and form an even cortex. The same formation occurs in some other sea-washed species; the arrangement of the tissue elements recalls that of crustaceous Florideae such as _Hildenbrandtia_, _Cruoria_, etc. [Illustration: Fig. 40. Young thallus of _Rhizocarpon geographicum_ DC., with primary and subsequent (dotted lines) areolation × 5.]