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Rh Phoronis has long been regarded as a possible ally of Rhabdopleura (see ); and Masterman (10) has attempted to demonstrate the existence in Actinotrocha of most of the structures which occur in the Pterobranchia. According to his view the praeoral hood of Actinotrocha (cf. fig. 4) corresponds with the “proboscis” of Pterobranchia; the succeeding region, as far as the bases of the tentacles, with the collar; and the post-tentacular region with the metasome. Masterman’s more detailed comparisons have for the most part been rejected by other morphologists. One of the most formidable difficulties in the way of the attempt to reduce Actinotrocha to the Pterobranchiate type of structure is the condition of the coelom in the former. There is indeed a perfectly definite transverse septum which divides the body-cavity in the region of the tentacle-bases. Even if it be admitted that the postseptal space may be the metasomatic cavity, the praeseptal space can hardly be regarded as coelomic in nature, since it is in continuity with the vascular system; while Masterman’s conclusion that the cavity of the praeoral hood (the supposed proboscis-cavity) is separated from that of the supposed collar has received no confirmation. In spite of these difficulties it must be conceded that the dorsal flexure of the alimentary canal of the Pterobranchia is very Phoronis-like. It has, moreover, been shown (see especially Goodrich, 5) that shortly before its metamorphosis, Actinotrocha develops a coelomic space which lies immediately in front of the oblique septum, and gives rise later to the cavity of the lophophore and tentacles. Regarding this as a collar-cavity, it becomes possible to agree with Masterman that the region shown in fig. 4, 1. between the tentacles and the praeoral hood, is really a collar the coelom of which develops relatively late. It will be noticed that the lophophore of Phoronis is, on this assumption, a derivative of the collar just as it is in the Pterobranchia. The epistome of the adult Phoronis cannot well be the proboscis since its cavity is continuous with the lophophoral coelom, and because the praeoral hood of Actinotrocha is entirely lost at the metamorphosis. It is possible that this consideration will account for the want of an anterior body-cavity in Phoronis. Since the proboscis is a purely larval organ in this genus it may be supposed that the coelomic space which properly belongs to it fails to develop, but that the praeoral hood itself is none the less the morphological representative of the proboscis. In spite of the criticisms which have been made on the conclusion that Phoronis is allied to the Pterobranchia, it is thus possible that the view is a sound one, and that the Phoronidea should take their place, with the Enteropneusta and the Pterobranchia, as an order of the Hemichordata.

.—(1) Benham, ''Quart. Journ. Mic. Soc.'' xxx. 125 (1890), (2) Caldwell, ''Proc. Roy. Soc.'' xxxiv. 371 (1883); (3) Cori, ''Zeitschr wiss. Zool.'' li. 480 (1891); (4) Fowler, art. “Hemichorda,” ''Ency. Brit.'' xxix. 249 (1902); (5) Goodrich, ''Quart. Journ. Mic. Soc.'' xlvii. 103 (1904); (6) Harmer, Siboga Rep. xxvi. 114, bis (Pterobranchia), (1905); (7) Ikeda, ''J. Coll. Sci. Japan'', xiii. 507 (1901); (8) Lankester, art. “,” ''Ency. Brit.'' xix. 430, 433 (1885); (9) De Selys-Longchamps, ''Arch. Biol.'' xviii. 495 (1902); Wiss. Meeresunt. (N. F.) vi. Abt Helgoland (1903), Heft i.; Mém classe ''sci. acad. belgique, vol i. (1904); Fauna u. Flora G. v. Neapet'', 30 Monogr. (1907); (10) Masterman, ''Quart Journ. Mic. Soc.'' xl. 281 (1898); xliii. 375 (1900); (11) Schultz, ''Zeitschr. wiss. Zool.'' lxxv. 391, 473 (1903); (12) Shearer, ''Mitth. zool. Stat. Neapel'', xvii. 487 (1906); 13) Shipley, Cambr. Nat. Hist. ii. 450 (1896).

 PHORORHACOS, the best-known genus of the extinct Patagonian Stereornithes (see : Fossil). Among the bones found in the strata of the Santa Cruz formation (now considered as mainly of mid-Miocene date) was the piece of a mandible which F. Ameghino described in 1887 as that of an edentate mammal, under the name of Phorysrhacos longissimus (Bolet. Mus. de la Plata, i. 24). In 1891 (Rev. Argent. Hist. Nat. i. 225) he amended the name and recognized the bone as that of a bird, Phororhacos, which with Brontornis and others constituted the family Phororhacidae. About six species of the type genus are now known, the most complete being ''Ph. inflatus'', with skull, mandible, pelvis, limbs and some of the vertebrae.

These birds were at first considered as either belonging to the Ratitae, or at least related to them, until C. W. Andrews, after much of the interesting material had been acquired by the British Museum, showed the gruiform affinities of Phororhacos (Ibis, 1896, pp. 1–12), a conclusion which he was able to further corroborate after the clearing of the adherent stony matrix from the skulls (Tr. Z. S. 1901, xv. pp. 55–86, pls. 14–17). The skull of ''Ph. longissimus'' is about 2 ft. long and 10 in. high; that of ''Ph. inflatus'' is 13 in. long, and this creature is supposed to have stood only 3 ft. high at the middle of the back. The under jaw is slightly curved upwards and it contains a large foramen as for instance in Psophia and in Mycteria. The strongly hooked upper beak is very high, and very much compressed laterally. The palate is imperfectly desmognathous, as in Dicholophus, with an inconspicuous vomer. The quadrate has a double knob for its articulation with the skull, and basipterygoid processes are absent. What l1ttle is known of the shoulder-girdle (breastbone still unknown) points to a flightless bird, and so do the short wing bones, although these are stout. The pelvis has an ischiadic foramen. The hind limbs are distinctly slender, the tibia of ''Ph. inflatus'' being between 15 and 16 in. in length.

For further detail see F. Ameghino, “Sur les oiseaux fossiles de la Patagonie,” ''Bolet. inst. geogr. argentino'', xv., chs. 11 and 12 (1895); F. P. Moreno and A. Mercerat, Catálogo de los pájaros fósiles de la República Argentina, ''An. Mus. La Plata'' (1891; with 21 plates).

 PHOSGENITE, a rare mineral consisting of lead chlorocarbonate, (PbCl)2CO3. The tetragonal (holosymmetric) crystals are prismatic or tabular in habit, and are bounded by smooth, bright faces: they are usually colourless and transparent, and have a brilliant adamantine lustre. Sometimes the crystals have a curious helical twist about the tetrad or principal axis. The hardness is 3 and the specific gravity 6.3. The mineral is rather sectile, and consequently was early known as “corneous lead” (Ger. Hornblei). The fanciful name phosgenite was given by A. Breithaupt in 1820, from phosgene, the old name of carbon oxychloride, because the mineral contains the elements carbon, oxygen and chlorine. At Cromford, near Matlock, it was long ago found in an old lead mine, being associated with anglesite and matlockite (Pb2OCl2) in cavities in decomposed galena: hence its common name cronfortite. Fine crystals are also found in galena at Monteponi near Iglesias in Sardinia, but the largest are those recently found near Dundas in Tasmania. Crystals of phosgenite, and also of the corresponding bromine compound [PbBr]2CO3, have been prepared artificially.

 PHOSPHATES, in chemistry, the name given to salts of phosphoric acid. As stated under, phosphoric oxide, P2O5, combines with water in three proportions to form H2O·P2O5 or HPO3, metaphosphoric acid; 2H2O·P2O5 or H4P2O7, pyrophosphoric acid; and 3H2O·P2O5 or H3PO4, orthophosphoric or ordinary phosphoric acid. These acids each give origin to several series of salts, those of ordinary phosphoric acid being the most important, and, in addition, are widely distributed in the mineral kingdom (see below under Mineral Phosphates).

Orthophosphoric acid, H3PO4, a tribasic acid, is obtained by boiling a solution of the pentoxide in water; by oxidizing red phosphorus with nitric acid, or yellow phosphorus under the surface of water by bromine or iodine, and also by decomposing a mineral phosphate with sulphuric acid. It usually forms a thin syrup which on concentration in a vacuum over sulphuric acid deposits hard, transparent, rhombic prisms which melt at 41.7°. On long heating the syrup is partially converted into pyrophosphoric and metaphosphoric acids, but on adding water and boiling the ortho-acid is re-formed. It gives origin to three classes of salts: M′H2PO4 or M″H4P2O8; M′2HPO4 or M″HPO4, M′3PO4, M″3P2O8 or M‴PO4, wherein M′, M″, M‴ denote a mono, di- and tri-valent metal. The first set may be called monometallic, the second bimetallic, and the third trimetallic salts. Per-acid salts of the alkalis, e.g. (K, Na, NH4)H5(PO4)2, are also known; these may be regarded as composed of a monometallic phosphate