PTEROPODA. 51<) phroditic. Limacina arctica is of the size of, and luoks like, a sweet-pea blossom, moving up and down in the water. It is common from Labrador to the polar regions. The largest form on the eastern coast of North .America is the beautiful PTOLEMAIC SYSTEM. FORMS OF PTEROPOD9. Clione pnpillionacea, which has a head and lin- gual ribbon. It is an inch long, the body fleshy, with no shell, the wings being rather small. It is likely to occur in vast numbers and forms the food of whales and other large surface- feeders. ( See Ooze. ) The larvfe of the ptero- pods pass through a trochosphere stage, spherical with a ciliated crown, and afterwards assume a veliger form. ( See Mollusca. ) The pteropods are, in some degree, a generalized type. They have a wide geographical distribution and a high anti<|uity. Consult authorities cited under Ga-stropoda; and Von Zittel and Eastman, Text- Book of Palaeontology (Xew York, 1900). PTEROSAURIA, ter'6-sa'ri-a (Xeo-Lat. nom. pi., from Gk. irTepSt, pteroii, feather, wing + aavpot. sauros. lizard). An order of extinct bird-like reptiles, with hollow bones, well-formed joints, and with the fore limbs modified for use as wings. See Ptebodacttl. PTER'YLO'SIS (Xeo-Lat., from ptenjla, feather-tract, from Gk. TrrepSv^ pteron. feather, wing + iXri, hylC; wood ) . The method of growth of feathers in birds. The bodies of most birds PTEBTL08I9. Dorsal and ventral views of the body of a quail, showing feather tracts. are not uniformly covered with feathers, such a condition being found only among the Ratitae and the penguins. In all other birds the feathers are grouped in clearly defined areas or tracts (pter^la-) with bare spaces (apteria) between tbem. To tliis condition, which has high taxono- mic value. Xitzsch i Pferi/lofiiaphie Ai , 11,11. /lalk. 1S40) gave the name -pteryiosis,' and to the aspect of the tracts "pterylography.' these tracts are different in the various groups, both in extent and position; but in general it may be said that feather-growth is fairly continuous over the head and throat; along the sides of the neck; on the shoulders, wings, tail, and lower part of the thighs, occasionally extending to the toes; in a band of varying breadth and shape down the centre of the back; and in two bands down the loner side of the body, where the central line from the throat to the vent is invariably free. It is believed that pteryiosis was not present in ancient birds, which primitively were feathered all over, but is an acquired trait conceived to be advantageous in economy of energy, and in mak- ing the feathers fit more snugly and lie better with reference to wind and rain. Consult a translation of Nitzsct's work, edited by Sdater and published by the Ray Society, London, in 1807 : also Gadow's article "Ptervlosis" in Xew- ton. Dictionnry of Birds (New York, 1893-96). PTINIDiE. A family of small beetles. See DE.ATH-A'ATCn. PTOLEMAIC (tol'e-ma'ik) SYSTEM. In astronomy, a planetary theory exjxmnded by Ptolemy (q.v. ). It was an attempt to reduce to a scientific form common and primitive no- tions concerning the motions of the heavenly bodies. The primar.v and fundamental doctrines of this system are that the earth is the centre of the universe, and that the heavenly bodies revolve round it in circles, and at a uniform rate. These notions, which are naturally sug- gested by the first general aspect of things, having, previous to any accurate observation, established themselves as unquestionable ax- ioms, phenomena which were found, on closer examination, to be inconsistent with them were explained by the introduction of additional hypotheses. In the Ptolemaic system, the earth, the most stable of the 'elements,' held the low- est place, and supported water, the second in order; above water was placed air, and then fire, ether being supposed to extend indefinitely above the others. In or beyond the ether element were certain zones or heavens, each heaven containing an immense crystalline spherical shell, the smallest inclosing the earth and its superincum- bent elements, and the larger spheres inclosing the smaller. To each of those spheres was at- tached a heavenly bodj', which, by the revolution of the crystalline, was made to move round the earth. The first or innermost sphere was that of the moon, and after it in order came those of jMercury. Venus, the sun, JIars. Jupiter, Saturn, and the fixed stars, eight in all. To this system later astronomers added a ninth sphere, the mo- tion of which should produce the precession (q.v.) of the equinoxes, and a tenth, to cause the alternation of day and night. This tenth sphere, or primum mobile, was supposed to revolve from east to west in 24 hours, and to carry the others along with it in its motion; but the Ptolemaic astronomers did not venture to explain how this was done, although since the axis of motion of the primum mobile was that of the equator, its extremities being the poles of the heavens, while that of the ninth sphere was the axis of the ecliptic, some explanation was certainly neces-
