PORRIGO. 



[ 629 ] 



rOTTIOIDEA. 



rORRI'CxO. See Favus. 



POTAMUCY'PKIS, Brady.— One of the 

 Cypridcc ; valves reniforiu, thick, right 

 larger than left ; upper antennte with very- 

 short set;« ; post-abdomiual rami rudimeii- 

 taiy. 1 British species. 



BiBL. Brady, Kat. Hist. Tr. North. 8,- 

 Durham, iii. 365. 



POTASH, AND ITS SALTS. 



Caustic Potash. — The strength of the 

 solution may be that of the Liq. Potasste 

 of the Pharmacopoeia. But we prefer a 

 stronger solution made with 1 drachm of 

 the potassa fnsa or stick-potash of the 

 shops, and 1 tluid oz. of water. The solu- 

 tion should be allowed to settle, and the 

 clear portion poured off into one of the 

 test-bottles (Intr. p. xxvii). 



Some remarks are made upon the action 

 of potash in the Inte., and others under 

 the heads of the tissues, kc. On treating 

 organic substances with this reagent, tlie 

 cvstic-oxide-like crystals of the carbonate 

 (PI. 10. fig. 2Q) will frequently be formed._ 



Chromates of Potash. — The bichromate is 

 used in the preparation of the chromate of 

 lead for injection. Its crystals polarize 

 well. The neutral chromate is also some- 

 times used for preparing injections. See 

 Peepaeation. 



Nitrate of potash, nitre, or saltpetre. — 

 This salt is dimorphous : it usually crystal- 

 lizes in six-sided prisms with dihedral sum- 

 mits, or in other forms belonging to the 

 right-rhombic prismatic system. But some- 

 times it assumes the form of obtuse rliom- 

 bohedra, resembling those of nitrate of soda, 

 and referable to the rhombohedric system. 



The crystals exhibit very beautifully the 

 phenomena of Analytic Ceystals. 



POTATO-FUNGUS. See Boteytis. 



POT'TIA, Ehr.— A genus of Pottiaceous 

 Mosses, including some of the Gymnostoma 

 and ire/ssi'tg of Hedwig and others. Wilson 

 separates as Anacalyptce the species with a 

 peristome (fig. GOO). 



Fig. 606. 



POTTIA'CE.^E.- 



Mosses. 



-A tribe of Po-tioi 



?3 



^"^mrnmm 



Pottia caespitosa. 

 Fragment of peristome. 

 Magnified 50 diametera. 



Synopsis of Genera, 



Pottia. Calyptra dimidiate. Peristome 

 simple or wanting ; if present, composed of 

 lanceolate articulate teeth, simple or with a 

 longitudinal line, rugulose and somewhat 

 fleshy. 



Trichosto7mitn. Calyptra dimidiate. Peri- 

 stome simple, sixteen teeth, each split to the 

 base into two cilia, or irregularly and there- 

 fore into more than two, erect, stifl^ and 

 not twisted. 



Barhula. Calyptra dimidiate-hood- 

 shaped. Peristome simple, ciliiform ; cilia 

 tliirty-two, solitary or approximated in 

 pairs" on a more or less exserted basilar 

 membrane, spUt into two cilioles behind, 

 very long, articulate-rugulose, twisted to 

 the left, rarely to the right, in one or several 

 spires, hygroscopic. Cells of the operculum 

 and calyptra twisted in the same way. 



Ceratodon. Calyptra dimidiate. Peri- 

 stome simple ; teeth sixteen, connate at the 

 base into a cellular membrane, split into 

 two long, nodosely articuUi.ted dark-coloured 

 arms, paler on each side, densely trabeculated 

 at the lower part. Capsule thick-skinned, 

 shining, nodding, with a somewhat nodose 

 collum ; annulate. 



Weissia. Calyptra dimidiate. Peristome 

 simple or wanting ; if present, composed of 

 sixteen lanceolate or subulate, entire or 

 cribrose, equidistant teeth. 



POTTIOI'DEyE.— A family of Acrocar- 

 pous operculate Mosses, but sometimes 

 Pleurocarpous by innovating branches. 

 Leaves of varied form, with a terete nerve ; 

 cells parenchymatous, hexagonal or squa- 

 rish six-sided, looser at the base, sometimes 

 very lax, more or less pellucid, often ex- 

 ceedingly transparent, large, fragile, rigid, 

 foraminate, bearing on the upper side soli- 

 tary papiUse or confluent papillte (hence 

 often truncate and tuberculate at the apex), 

 placed in the middle of the cell ; cells 

 mostly fuU of chlorophyll, often very small 

 and thickened. Capsule erect, rarely in- 

 clined, oval, elliptic or pear-shaped oblong, 

 smooth or striate, the operculum mostly- 

 conical or beaked. 



This family is divided into three tribes : 

 Calympeeace^. Basilar cells of the 

 leaves rigid, hyaline, often very brittle, 

 more or less ample, empty, distinctly fora- 

 minated. 



PoTTiACEiE. Basilar cells of the leaves 



