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 Limbs stout, and of moderate length. Three completely developed toes, with distinct broad rounded hoofs on each foot. Teats two, inguinal. Eyes small. Ears of moderate size, oval, erect, prominent, placed near the occiput. Skin very thick, in many species thrown into massive folds. Hairy covering scanty. One or two median horns on the face. When one is present it is situated over the conjoined nasal bones; when two, the hinder one is over the frontals. These horns, which are of a more or less conical form and usually recurved, and often grow to a great length (three or even four feet), are composed of a solid mass of hardened epidermic cells growing from a cluster of long dermal papillae. The cells formed on each papilla constitute a distinct horny fibre, like a thick hair, and the whole is cemented together by an intermediate mass of cells which grow up from the interspaces between the papillae. It results from this that the horn has the appearance of a mass of agglutinated hairs, which, in the newly growing part at the base, readily fray out on destruction of the softer intermediate substance, but the fibres differ from true hairs in growing from a free papilla of the derm, and not within a follicular involution of the same. Considerable difference of opinion exists with regard to the best classification of the family, some authorities including most of the species in the typical genus Rhinoceros, while others recognize quite a number of sub-families and still more genera. Here the family is divided into two groups Rhinocerotinae and Elasmotheriinae, the latter including only Elasmotherium, and the former all the rest. In the Lower Oligocene of Europe we have Ronzotherium and in that of America Leptaceratherium (Trigonias), which were primitive species with persistent upper canines and three-toed fore-feet. Possibly they belonged to the Amynodontidae, but they may have been related to the Upper Oligocene Diceratherium, in which the nasal bones formed a transverse pair; this genus being common to Europe and North America. Caenopus is an allied American type. Hornless rhinoceroses, with five front-toes, ranging from the Oligocene to the Lower Pliocene in Europe, represent the genus Aceratherium, which may also occur in America, as it certainly does in India. With the short-skulled, short-footed, three-toed and generally horned rhinoceroses ranging in Europe and America from the Lower Miocene to the Lower Pliocene, typified by the European R. goldfursi and R. brachypus, we may consider the genus Rhinoceros to commence; these species constituting the subgenus Teleoceras. The living R. (Dicerorhinus) sumatrensis of south-eastern Asia indicates another subgenus, represented in the European Miocene by R. sansaniensis and in the Indian Pliocene by R. platyrhinus, in which two horns are combined with the presence of upper incisors and lower canines. Next we have the living African species, representing the subgenus Diceros, in which there are two horns but no front teeth. To this group belongs the extinct European and Asiatic woolly rhinoceros, Rhinoceros (Diceros) antiquitatis, of Pleistocene age, of which the frozen bodies are sometimes found in Siberia, and R. (D.) pachygnathus of the Lower Pliocene of Greece. Finally the Great Indian rhinoceros R. unicornis, the Javan R. sondaicus, and the Lower Pliocene Indian R. sivalensis and R. palaeindicus, represent Rhinoceros proper, in which front teeth are present, but there is only one horn. (See ).

The subfamily Elasmotheriinae is represented only by the huge E. sibircum of the Siberian Pleistocene, in which the premolars were reduced to while front-teeth were probably wanting, and the cheek teeth developed tall crowns, without roots, but with cement in the valleys, and the enamel of the central parts curiously crimped. A hump on the forehead probably indicates the existence of a large frontal horn.

.—J. L. Wortman and C. Earle, “Ancestors of the Tapir from the Lower Miocene of Dakota,” ''Bull. Amer. Mus.'' vol. v. art. 11. (1893); H. F. Osborn, “Phylogeny of the Rhinoceroses of Europe,” op. cit. vol. xiii art. 19 (1900); O. Thomas, “Notes on the Type Specimen of Rhinoceros lasiotis, with Remarks on the Generic Position of the Living Species of Rhinoceros,” Proc. ''Zool. Soc''. (London, 1901).

 PERISTYLE (Gr., round, and , column), in architecture, a range of columns (whether rectangular or circular on plan) in one or two rows, enclosing the sanctuary of a temple; the term is also applied to the same feature when built round the court in which the temple is situated and in Roman houses to the court in the rear, round which the private rooms of the family were arranged, which were entered from the covered colonnade round the court.  PERITONITIS, inflammation of the peritoneum—the serous membrane which lines the abdominal and pelvic cavities and gives a covering to their viscera. It may exist in an acute or a chronic form, and may be either localized or diffused.

Acute peritonitis may be brought on, like other inflammations, by exposure to wet or cold, or in connexion with injury to, or disease of, some abdominal organ, or with general feebleness of health. It is an occasional result of hernia and of obstruction

of the bowels, of wounds penetrating the abdomen, of the perforation of viscera, as in ulcer of the stomach, and of the intestine in typhoid fever, of the bursting of abscesses or cysts into the abdominal cavity, and also of the extensions of inflammatory action from some abdominal or pelvic organ, such as the appendix, the uterus, or bladder. At first localized, it may afterwards become general. The changes effected in the peritoneum are similar to those undergone by other serous membranes when inflamed. Thus, there are congestion; exudation of lymph in greater or less abundance, at first greyish and soft, thereafter yellow, becoming tough and causing the folds of the intestine to adhere together; effusion of fluid, either clear, turbid, bloody or purulent. The tough, plastic lymph connecting adjacent folds of intestine is sometimes drawn out like spun-glass by the movements of the intestines, forming bands and loops through or beneath which a piece of bowel may become fatally snared.

The symptoms of acute peritonitis usually begin by a shivering fit or rigor, together with vomiting, and with pain in the abdomen of a peculiarly severe and sickening character, accompanied with extreme tenderness, so that pressure, even of the bed-clothes, causes aggravation of suffering. The patient lies on the back with the knees drawn up so as to relax the abdominal muscles; the breathing becomes rapid and shallow, and is performed by movements of the chest only, the abdominal muscles remaining quiescent—unlike what takes place in healthy respiration. The abdomen becomes swollen by flatulent distension of the intestines, which increases the distress. There is usually constipation. The skin is hot, although there may be perspiration; the pulse is small, hard and wiry; the urine is scanty and high coloured, and is passed with pain. The face is pinched and anxious. These symptoms may pass off in a day or two; if they do not the case is apt to go on to a fatal termination. In such event the abdomen becomes more distended; hiccough, and the vomiting of brown or blood-coloured matter occur; the temperature falls, the face becomes cold and clammy; the pulse is exceedingly rapid and feeble, and death takes place from collapse, the mental faculties remaining clear till the close. When the peritonitis is due to perforation—as may happen in the case of gastric ulcer or of ulcers of typhoid fever, or in the giving way of a loop of strangulated bowel—the above-mentioned symptoms and the fatal collapse may all take place in from twelve to twenty-four hours. The puerperal form of this disease, which comes on within a day or two after childbirth, is often rapidly fatal. The actual cause of death is the absorption of the poisonous inflammatory products which have been poured out into the peritoneal cavity, as well as of the toxic fluids which have remained stagnant in the paralysed bowel.

Perhaps the commonest cause of septic peritonitis is the escape of micro-organisms (bacillus coli) from the ulcerated, mortified or inflamed appendix (see ). A generation or so ago deaths from this cause were generally placed under the single heading of “peritonitis,” but at the present time the primary disease is shown upon the certificate which too often runs thus: appendicitis five days, acute peritonitis two days.

Chronic peritonitis may occur as a result of the acute attack, or as a tuberculous disease. In the former case, the gravest symptoms having subsided, some abdominal pain continues, and there is considerable swelling of the abdomen, corresponding to a thickening of the peritoneum, and to the presence of fluid in the peritoneal cavity. This kind of peritonitis may also develop slowly without there having been any preceding acute attack. There is a gradual loss of strength and flesh. The disease is essentially a chronic one; it is not usually fatal.

Tuberculons peritonitis occurs either alone or in association with tuberculous disease of a joint or of the lungs. The chief symptoms are abdominal discomfort, or pain, and distension of the bowels. The patient may suffer from either constipation or diarrhoea, or each alternately. Along with these local manifestations there may exist the usual phenomena of tuberculous disease, viz. high fever, with rapid emaciation and loss of strength. But some cases of tuberculous peritonitis present symptoms which are not only obscure, but actually misleading.