PETALOTRICHA. 



PEZIZA. 



face, but expanding at the end into a delicate 

 film. The film and pseudopodia become 

 globular before retraction. 



P. diffluent (PI. 52. fig. 16), freshwater. 



BIBL. Claparede et Lachmann, Etudes, 

 442. 



PETALOT'RICHA, K A genus of 

 marine Peritrichous Infusoria. Free, at- 

 tached to the base of a horny carapace by 

 a peduncle j anteriorly a ring of ciliated 

 petaloid segments. 2 species. (Kent, Inf. 

 627.) 



PETALS. The petals of Flowering 

 Plants afford many interesting microscopic 

 objects, in the epidermis, glandular and 

 other hairs, the colour-cells, and the veins 

 composed of spiral vessels. Entire petals 

 of small size and delicate character form 

 good objects when dried and mounted in 

 Canada balsam. Those of the smaller 

 Caryophyllaceae, the ligulate corollas of 

 Composite, &c., are well suited for this. 

 The larger kinds are studied by means of 

 sections, like LEAVES. 



PETROBIA, Murr.,ss?Wfanye&tf8 cris- 

 fatus. 



PETRO'BIUS, Leach. A genus of In- 

 sects, of the order Thysanura, and family 

 Lepismenae. 



P. maritimm has a general resemblance 

 to Lepisma saccharina ; but it exercises a 

 leaping movement. The antennae are longer 

 than the body ; of the setae at the tail, the 

 middle one is longest. The insect is of a 

 blackish-brown colour, and is covered with 

 scales ; the legs are yellowish, and the caudal 

 setae ringed with white ; the abdomen is 

 furnished will gill-like processes. 



It is found upon the rocky sea-coast. 



The scales have been used as test-objects. 



BIBL. Gervais, Wakkenaer's Apt. iii. 

 447; Guerin, Iconogr. Ins. pi. 2. fig. Ijf; 

 and Ann. Sc. Nat. 2 ser. v. 374. 



PETRONE'MA, Thwaites. A genus of 

 Oscillatoriaceae (Confervoid Algae). P. 

 fnth'culosa grows as a frustulose olive- 

 'brown crust on limestone rocks (not marine), 

 forming little hemispherical masses ; the 

 sheaths are thick and cartilaginous, brown 

 above but colourless at the tips, the proto- 

 plasm dull green. 



BIBL. Engl. Bot. Supp. pi. 2959. 



PETER'S GLANDS. See INTESTINES 

 (p. 440). 



PEYSSONELIA, Dene. A genus of 

 Cryptonemiaceee (Florideous Algae), con- 

 sisting of small plants with a depressed 

 lobed tlialliis (fig. 560), growing overstones, 



shells, &.c., attached by the whole under 

 surface, which produces jointed radical hairs 

 (fig. 561), especially at the thin margins. 



Fig. 560. 



Thallus. Nat. size. 



Fig. 561. 



Peyssonelia squamosa. 



Vertical section of a portion through two warts. 

 Magnified 25 diameters. 



The thallus is composed of several rows of 

 compact parenchymatous cells, and bears 

 on the concentrically-marked surface warts 

 composed of radiating rows of cells, among 

 which occur crucially-divided tetraspores. 

 P. Dubyi is not uncommon on British 

 shores j it is 1 to 2" in diameter, roundish 

 at first, ultimately irregularly lobed, and 

 dull brownish. Thuret has observed an- 

 theridia on distinct plants of P. squamosa, a 

 Mediterranean form j they are jointed fila- 

 ments collected into wart-like bodies, like 

 those containing the tetraspores. 



BIBL. Harvey, Mar. Ala. 144, pi. 14 D; 

 Phyc. Brit. pi. '71 j Thuret, Ann. Sc. Nat. 

 4 ser. iii. 23, pi. 4. 



PEZI'ZA, Dill. A genus of HelveUacei 

 (Ascomycetous Fungi), containing nume- 

 rous species, a large number of which grow 

 upon dead wood, on the ground, among 

 leaves c., many brightly coloured. They 

 are at first closed sacs, which burst at the 

 summit, and spread out to form a kind of 

 cup containing asci and paraphyses. Thus 

 they belong to the Discomycetes of some 

 anthors. 



Tulasne has shown that some of the 

 have a secondary fructification con- 



