476 



PLANTAGINEA PLEUROBRANCHUS. 



naeus, and the propriety of so doing is manifest from 

 an investigation of the animal constructing this shell. 

 If no other reason existed for the separation being 

 made, it would have been desirable in order to distin- 

 guish the terrestrial from the aquatic species, which 

 were indiscriminately blended. This genus inhabits 

 fre*h water ; the form of the shell is discoid, that is, 

 having its spiral evolutions on a horizontal plane, or, 

 as it were, wound round a central point, gradually 

 increasing in size v and leaving the upper and lower 

 sides concave and nearly similar, the spire being only 

 but slightly elevated ; the aperture is oval, dilated, and 

 far remove'd from the axis of the shell ; the lip is never 

 reflected, the substance is generally thin and dia- 

 phanous, the whorls nearly cylindrical, but sometimes 

 flattened, angular, or carinated ; they possess no oper- 

 culum. Lamarck does not appear to have observed 

 in more than one species of the Planorbis spirorbis 

 the very peculiar and characteristic distinction of its 

 being heterostrophe or left-handed, commonly called 

 a reverse shell, which also occurs in many other ex- 

 amples examined by the writer. This genus may be 

 divided into two species, such as are keeled or cari- 

 nated, and those not so ; the Planorbis corneus is a 

 specimen of the latter, and the P. carinatus of the 

 former. The species of this genus inhabit the lakes 

 and ditches of Europe. Some are known in a fossil state. 



PLANTAGINE^E. A small natural order of 

 herbs, containing only three genera and eighty spe- 

 cies. The species are, for the most part, mere weeds, 

 found on waste ground all over the world. The 

 genera are Plantago, LUtorella, and Glaux. The 

 leaves are stellate, and occasionally ternate : the 

 pubescence is jointed ; the flowers are brownish, and 

 arranged in dense spikes. Their leaves are rather 

 bitter and astringent ; their seeds mucilaginous and 

 rather acrid ; those of P. arenaria are imported from 

 the south of France in great quantities for the pur- 

 pose of forming an infusion in which muslins are 

 washed. P. lanceolata is an agricultural plant, being 

 sown among other grasses in laying down sheep pas- 

 tures, and sold by seedsmen as rib-grass. Hort. Brit. 



The plantains have a tendency to follow the mi- 

 grations of man ; and have among the natives of some 

 of our own foreign settlements been named the 

 " Englishman's foot," for wherever it is found there 

 our countrymen have trod. The LUtorella, is the 

 shore-weed of British botany, and the Glaux maritima 

 is the black salt-wort of the same, 



PLANTANTHERA (Richards). A genus of 

 three species of plants belonging to Orchidaceee. The 

 P. bifolia of Richards is the same species called Orchis 

 bifolia, or butterfly orchis, of Linnaeus. 



PLANULACEA, in modern malacology, is the 

 second family of the first class Cephalophora, first, 

 order Cryptndibranchiata ; it includes the genera 

 Kcnullna and Pcneroplis. 



PLANULARIA is a genus established by De 

 France to a fossil mollusc, now united to the genus 

 Peneroplis. 



PLATANUS (Linnaeus). A genus of highly 

 ornamental timber-tree?, natives of the Levant and of 

 North America. Their flowers are inconspicuous 

 and monoecious, and belong to Amentaceee. The 

 species are known by the common name of plane- 

 trees, and both the P. orientalis and ucridentalis are 

 elegant, and, when full grown, majestic plants. They 

 are usually propagated by cuttings planted in a shady 

 border in autumn. 



PLATINUM. This mineral was entirely unknown 

 to the ancients, and we believe that the first known 

 specimen came to this country about the middle of the 

 last century. This valuable metal is usually found in 

 grains combined with gold, iron, lead, palladium, rho- 

 dium, iridimn, and osmium, and the largest mass ever 

 discovered weighed less than twenty-eight ounces. 

 Its main place of deposit is a vast alluvial tract in 

 New Granada, South America. The valuable pro- 

 perty that platinum possesses of remaining unaltered 

 in the air, or when exposed to the action of heat at a 

 high temperature in the furnace, peculiarly fits it for 

 a variety of purposes in the useful arts ; but the great 

 expense attendant on its manufacture has hitherto 

 prevented its employment for domestic purposes, if 

 at least we except its application in ornamenting 

 porcelain. 



PL ATYLOBIUM (Smith). A genus of Austra- 

 lian evergreen shrubs, belonging to Leguminoste. 

 From the compressed form of their pods they are 

 vulgarly called the flat pea. They are handsome 

 greenhouse plants, thrive well in light loam and heath 

 mould, and are easily increased by cuttings. 



PLATYPUS (Herbst). A genus of xylophagous 

 beetles, belonging to the family Bostricliidae, and dis- 

 tinguished by having the antennae short, terminated 

 by a very large club, which is destitute of joints ; the 

 body is linear, the head truncate in front, the tibia; 

 broad and curiously striated, and the tarsi long and 

 very slender ; the penultimate joint not being bilobed. 

 These are curious little wood-boring insects, of which 

 but few species are known, the type is the P. cylin- 

 drus of Herbst, about a quarter of an inch long, which 

 has occurred in the New Forest in Hampshire. There 

 are eight or nine other exotic species. 



PLATYRHINUS (Clairville ; MACROCEPHALUS, 

 Olivier). A genus of coleopterous insects, belonging 

 to the section Tetramera, and sub-section Rhyncophora, 

 or weevils (see CURCULIONIDES), and sub-family 

 Anthribides; the antennae are clavate, the rostrum 

 short and broad, the eyes entire, the body of an ob- 

 long form, and the two basal joints of the antenna; 

 short. This genus comprises but a single British 

 species Curculio latirostris of Bonsdorff, which is half 

 an inch long, and is found in Sphceria fmxinea, 

 growing on the trunks of various trees. The other 

 genera, which belong to this sub-family (which was 

 omitted in its place) are Anthribus, Rhinomacer, Tro- 

 pidercs, Brashytarsus, and Phlocobius, all of which are 

 British, together with a great number of genera de- 

 scribed by Schonherr in his work in course of pub- 

 lication upon the Curculiomdee. 



PLECTRANTHUS (Heritier), A genus of 

 annuals and undershrubs natives of various parts of 

 the globe. Their flowers are for the most part blue 

 and didynamous, of course belonging to Labiates. 

 The shrubby sorts are easily kept and propagated, 

 but are plants of no beauty. 



PLEUROBRANCH1DIUM. A naked mol- 

 lusc without any trace of shell, occupying its place 

 as a connecting link between the families Subaply- 

 siacea and Aplysiacea in modern malacology: 



PLEURObRANCHUS (Cuvier). The sheila 

 belonging to this genus are but little known, and the 

 P. poranii is perhaps the only example described. 

 The form is large, well shaped, the edges mem- 

 branous, oval, and concave beneath, convex above, 

 the sharp edges closely united, and the summit alto- 

 gether subspiral posteriorly. 



