222 p L E P L E land. The sharp and slender teeth would be admirably adapted for catching and holding a slippery prey, and there is no doubt that fishes formed in part, if not altogether, its natural food. Indeed the scales and teeth of fishes have been found, in one case at least, just below the vertebra?, in the region which must have been occupied by the creature s stomach. It is of interest to note the differences which exist between the Plcsiosaurus and the ICHTHYOSAURUS (q.v. ), the latter being the type of another group, the Ichthyosauria, which is by some palaeontolo gists included with the Plcsiosauria in a larger group called Enalio- sauria. In outward form the Ichthyosaurus must have resembled some of the recent Cetacea, inasmuch as the head is proportionately large, and without any appearance of a neck joins directly on to the trunk. The hind limbs are smaller than the front ones, and the bones of both limbs are much more shortened and flattened than in Plcsiosaurus ; in addition to this there are supernumerary rows of bones, besides the five typical digital series. The pectoral arch (litters in having distinct clavicles and interclavicles. The vertebrae are short from back to front, deeply biconcave, and their neural arches never have a bony connexion with the centra. There is no sacrum. The teeth are placed in a groove, and not in separate sockets. The eye-ball was protected by a series of bony sclerotic plates, which are not found in Plcsiosaurus. The group Plcsiosauria includes several other genera besides the Plesiosaurus ; but most of these are only represented by such imperfect specimens that the distinctions between them, as at present known, are far from satisfactory. The characters which have been relied upon for their separation are to be found chiefly in the structure of the pectoral arch, limbs, and vertebra 1 . Plcsiosaurus is only certainly known to have existed from the time of the Lower Lias to the Chalk ; and it is especially characteristic of the Lias. More than fifty species, sometimes placed in several subgenera, have been described from different localities in Britain, some of which are represented by remarkably perfect specimens, and others by fragments only. This genus has a wide geographical distribu tion, species having been named from Secondary strata, on the con tinent of Europe, in India, Australia, South America, and North America. The closely allied and gigantic fonnPliosauruais chiefly characteristic of the English Oolites. In European Triassic beds, Plcsiosauria are represented by such genera as Nothosaurus, Simosaurus, and Pistosaurus, in all of which the neural arches seem to have been less closely united to the vertebral centra than in Plcsiosaurus. Ncusticosaurus is another Triassic form, remarkable, not only on account of its small size, being less than 12 inches in length, but also because its limbs seem to show a transitional condition ; for, while the structure of the hind limb resembles that of a land reptile, the fore limb seems to have had more the structure of a paddle. A number of forms closely related to the Plcsiosaunes have been described from rocks of Cretaceous age in North America under the following generic names Cimoliasaurus, Elasmosaurus, Oligo- simus, Piratosaurus, and Polycotylus. Of these the Elasmosaurus is better known than any of the others. It was an extremely elon gated form, as may be gathered from the fact that the snake-like neck alone consisted of more than sixty vertebra, the entire body, as we have noticed above, being more than 45 feet in length. Sec Conybcare, Trans. Geol. Soc., ser. 1, vol. v. p. 559, 1821, and ser. 2, vol. i. p. 103, 1824; Owen, Brit. Assoc. Rep., 1839, p. 43; Hawkins, Great Sea Dragons, 1840; Phillips, Valley of the Thames, 1871; Huxley, Anal, of Vert. Anim., 1871, p. 208; Nicholson, Palxontoloyy, vol. ii. p. 218, 1879; Sollas, Quart. Journ. Geol. Soc., vol. xxxviii. p. 440, 1881 ; Hulke, Presidential Address, Geol. Soc., 1883 ; Lcidy, "Fossil Vertebrates," in Report U. S. Geol. Surv. Terri tories, vol. i., 1873 ; and Cope, ibid., vol. ii., 1875. (E. T. N.) PLETHO. See GEMISTUS. PLEURISY, or PLEUEITIS, inflammation of the pleura or serous membrane investing the lungs and lining the interior of the thoracic cavity. It is a common form of chest complaint, and may be either acute or chronic, more frequently the former. The morbid changes which the pleura undergoes when inflamed are similar to those which take place in other serous membranes, such as the peritoneum (see PERITONI TIS), and consist of three chief conditions or stages of pro gress. (1) Inflammatory congestion and infiltration of the pleura, which may spread to the tissues of the lung on the one hand, and to those of the chest wall on the other. (2) Exudation of lymph on the pleural surfaces. This lymph is of variable consistence, sometimes composed of thin and easily separated pellicles, or of extensive thick masses or strata, or again showing itself in the form of a tough membrane. It is of greyish -yellow colour, and microscopically consists mainly of coagulated fibrine along with epithelial cells and red and white blood corpuscles. Its presence causes roughening of the two pleural surfaces, which, slightly separated in health, may now be brought into contact by bands of lymph extending between them. These bands may break up or may become organized by the development of new blood-vessels, and adhering per manently may obliterate throughout a greater or less space the pleural sac, and interfere to some extent with the free play of the lungs. (3) Effusion of fluid into the pleural cavity. This fluid may vary in its characters. Most commonly it is clear or slightly turbid, of yellowish-green colour, sero-fibrinous, and containing flocculi of lymph. In bad constitutions or in cases where the pleurisy complicates" some severe form of disease, e.y., the acute infectious maladies, it is deeply-coloured, bile-stained, sero-purulent, purulent, or bloody, occasionally containing bubbles of air from decomposition. The amount may vary from an almost inappreciable quantity to a gallon or more. When large in quantity it may fill to distension the pleural sac, bulge out the thoracic wall externally, and compress more or less completely the lung, which may in such cases have all its air displaced and be reduced to a mere fraction of its natural bulk lying squeezed up upon its own root. Other organs, such as the heart and liver, may in conse quence of the presence of the fluid be shifted away from their normal position. In favourable cases the fluid is absorbed more or less completely and the pleural surfaces again may unite by adhesions ; or, all traces of inflam matory products -having disappeared, the pleura may be restored to its normal condition. When the fluid is not speedily absorbed it may remain long in the cavity and compress the lung to such a degree as to render it incapable of re-expansion as the effusion passes slowly away. The consequence is that the chest wall falls in, the ribs become approximated, the shoulder is lowered, the spine becomes curved and internal organs permanently displaced, while the affected side scarcely moves in respiration. Some times the unabsorbed fluid becomes purulent, and an emj)ysema is the result. In such a case the matter seeks vent in some direction, and it may point as an abscess upon the chest or abdominal wall, or on the other hand burst into the lung and be discharged by the mouth. It must be observed that many cases of pleurisy do not reach the stage of effusion, the inflammation terminating with the exudation of lymph. To this form the term dry pleurisy is applied. Further pleurisy may be limited to a very small area, or, on the contrary, may affect throughout a greater or less extent the pleural surfaces of both lungs. Pleurisy frequently arises from exposure to cold ; hence it is more common in the colder weather ; but besides this various other causes are connected with its occurrence. Thus it is often associated with other forms of disease within the chest, more particularly pneumonia, bronchitis, and phthisis, and also occasionally accompanies pericarditis. Again it is apt to occur as a secondary disease in certain morbid constitutional states, e.g., the infectious fevers, rheumatism, gout, Bright s disease, diabetes, &c. Further, wounds or injuries of the thoracic walls are apt to set up pleurisy, and the rupture of a phthisical cavity in the lungs causing the escape of air and matter into the pleura has usually a similar effect. The symptoms of pleurisy vary, being generally well- marked, but sometimes obscure. In the case of dry pleurisy, which is on the whole the milder form, the chief symptom is a sharp pain in the side, felt especially in breathing. Fever may or may not be present. There is slight dry cOugh ; the breathing is quicker than natural, and is shallow and of catching character. If much pain
is present the body leans somewhat to the affected side, to