PETALOTRICHA. 



[ 589 ] 



PEZIZA. 



face, but expanding at the end into a delicate 

 film. The film and pseudopodia become 

 globular before retraction. 



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



BiBL. Claparede et Lachniann, Etndes, 

 44i>. 



PETALOT'RICHA, K.— A genus of 

 marine Peritrichous Infusoria. Free, at- 

 tached to tlie base of a horny carapace by 

 a peduncle ; 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 

 Caiyophyllacese, the ligulate corollas of 

 Composita?, Szc, are well suited for this. 

 The larger kinds are studied by means of 

 sections, like Leaves. 



PETROBIA, 'Muvv.,= Tefrani/chu.i oris- 

 fatus. 



PETRO'BIUS, Leach.— A genus of Li- 

 sects, of the order Thysauura, and family 

 Lepismenas. 



P. }nan'ti}nus has a general resemblance 

 to Lepisma saccharina ; but it exercises a 

 leaping movement. The antennee are longer 

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

 middle one is longest. The insect is of a 

 blackish-brown colour, and is covered with 

 scales ; the legs ai-e yellowish , and the caudal 

 setae ringed Avith white ; the abdomen is 

 furnished Avill gill-like processes. 



It is foimd upon the rocky- sea-coast. 



The scales have been used as test-objects. 



BiBL. Gervais, Walckenaer's Apt. iii. 

 447; Guerin, Iconogr. Ins. pi. 2. tig. If; 

 and Ami. Sc. Kat. 2 ser. v. 374. 



PETRONE'MA, Thwaites.— A genus of 

 Oscillatoriacefe (Confervoid Algae). P. 

 frvticuhsa 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. 



PEYER'S GLANDS. See Lctestine.s 

 (p. 440). 



PEYSSONELIA, Dene.— A genus of 

 C.i-j-ptonemiacese (Florideous Algae), con- 

 sisting of small plants with a depressed 

 lobed ihallus (fig. 560), growing over stones. 



shells, &c., attached by the whole under 

 surface, which produces jointed radical hairs 

 (fig. 661), 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 conceutrically-marlved surface warts 

 composed of radiating rows of cells, among 

 which occur crucially-divided tetraspores. 

 P. Dubyi is not uncommon on British 

 shores ; 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 ; they are jointed fila- 

 ments collected into wart-like bodies, like 

 those containing the tetraspores. 



BiBL. Harvev, Mar. Ah/. 144, pi. 14 D; 

 P7ii/c. Brit. pi. '71 ; Thuret, Ann. Sc. Nat. 

 4 ser. iii. 23, pi. 4. 



PEZI'ZA, Dill.— A genus (.f HelveUacei 

 ( Ascomycetous . Fungi), containing nume- 

 rous species, a large number of Avhich grow 

 upon dead wood, on the ground, among 

 leaves kc, 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 

 authors. 



Tulasne has shown that some of the 

 Pezizce have a secondarv fructification con- 



