PARMELIACE^. 



[ 493 ] 



PEDIASTRUM. 



without a raised border, but ciliated at the 



margins 



If. EvERNiA. Thallus somewhat crus- 

 taceous, branched and laciniated, angled or 

 compressed, cottony within. ApotJiecia cir- 

 cular, shield-shaped, sessile, with the disk 

 concave, coloured, and an inflexed border 

 formed b)" the thallus. 



III. Alectoria. Thallus cartilaginous, 

 somewhat thready, branched, prostrate or 

 pendulous, somewhat fistulose and cottony 

 within. Apothecia circular, thick, sessile, 

 plane or convex, more or less bordered, 

 entirely formed of the thallus, and of the 

 same colour. 



IV. CoRNicuLARiA. TJidllus Cartilagi- 

 nous, branched, subcylindrical, nearly solid 

 and cottony within. Apothecia circular, 

 terminal, obliquely peltate, entirely formed 

 of the substance of the thallus, at length 

 convex, more or less bordered and often 

 toothed. 



V. Ramalixa. Thallus cartilaginous, 

 generally branched and laciniated, somewhat 

 shrubby, generally bearing powdery warts, 

 cottony and compact within. Apothecia cir- 

 cular, shield-shaped, stalked and peltate, flat, 

 bordered, entirely formed of the substance 

 of the thallus, and mostly of the same colour. 



VI. RoccELLA. Thallus cartilaginous, 

 leathery, rounded or flat, branched or laci- 

 niated. Apothecia cncular, adnate to the 

 thallus, the disk coloured, plano-convex, 

 with a border, at length thickened and ele- 

 vated, formed of the thallus, and covering a 

 black powder concealed within the substance 

 of the thallus. 



VII. Cetraria. Thallus foliaceous, 

 cartilagineo - membranous, ascending or 

 spreading, lobed and laciniated, smooth and 

 naked on both sides. Apothecia circular, 

 obliquely adnate to the margin of the 

 thallus, the lower portion being free (from 

 the thallus) ; disk coloured, plano-concave, 

 with an inflexed border formed of the 

 thallus. 



VIII. Nephroma. Thallus foliaceous, 

 leathery or membranous, spreading, lobed, 

 naked or hairy beneath. Apothecia circular 

 or reniform, adnate on the underside of the 

 lobules of the thallus, with a border formed 

 by the latter. 



IX. Peltigera. Thallus foliaceous, 

 leathery or membranous, spreading, lobed, 

 with woolly veins beneath. Apothecia some- 

 what circular, adnate on the upper side of 

 the lobules of the thallus, and having a 

 border formed bv this. 



X. Sticta. Thallus foliaceous, leathery- 

 cartilaginous, spreading, lobed, free and 

 downy beneath, with little cavities or hollow 

 spots, often containing a powdery substance. 

 Apothecia beneath formed of the thallus, to 

 which they are appressed and fixed by a 

 central point, the disk coloured, flat, sur- 

 rounded by an elevated border formed of the 

 thallus. 



XI. Parmelia. Thallus foliaceous, mem- 

 branous or leathery, spreading, lobed and 

 stellated or laciniated, more or less fibrous 

 beneath. Apothecia circular, formed by the 

 thallus, fixed by a central point, disk con- 

 cave, coloured with an inflexed margin from 

 the thallus. 



XII. Borrera. Thallus cartilaginous, 

 branched and laciniated, the segments fi-ee, 

 generally grooved beneath, the margins fre- 

 quentl}'^ ciliated. Apothecia circular, peltate, 

 formed of the thallus, the disk coloured and 

 surrounded by an inflexed margin derived 

 from the thallus. 



XIII. Lecanora. T^aZ/M5 crustaceous, 

 spreading, flat, adnate and uniform. Apo- 

 thecia circular, thick, sessile and adnate ; 

 disk plano-convex, the border thickish, 

 formed of the crust, and of the same colour. 



BiBL. See the genera. 



PASTE, Eels in. See Anguillula. 



PEARLS. — These well-known bodies are 

 formed as secretions from the mantle of 

 bivalve mollusks ; the best being obtained 

 from the Ceylon pearl-oyster or mussel 

 {Avicula margaritifera). They occur natu- 

 rally from the irritation produced by particles 

 of sand accidentally confined between the 

 mantle and the shell] and they are produced 

 artificially by wounding the mantle with 

 pieces of iron-wire, &c. Their structure 

 agrees with that of the shell of the animal in 

 which they are formed. Sometimes they 

 consist entirely of nacre or pearly matter, 

 arranged in close concentric layers ; at 

 others, the interior exhibits the prismatic 

 structure of shell. 



When acted upon by a dilute mineral acid, 

 the lime-salt is removed from the organic 

 cast of the original, which is left. 



See Shell. 



PEDIASTRUM, Meyen.— A genus of 

 Desmidiacea3 (Confervoid Algae). 



Char. Cells aggregated into a usually 

 circular, minute disk or flattened star, and 

 generally arranged either in a single or in 

 two or more concentric series ; marginal 

 cells bipartite on the outside. 



Ralfs describes eleven British species. 



