AN ODONTOLOGICAL STUDY ON CRAB-EATING MONKEY

Abstract

Morphology of the teeth of Primates and the related literatures thereof are the most essential tools for any one who undertakes to study the phylogenetic aspect ofthe human tooth form. Unfortunately, such studies are available so far to a very limited extent, especially when it deals with teeth of the genus Macaca. The present paper is to furnish some of the knowledges on the tooth forms and morphology of the dental arch of the crab-eating monkey which were obtained from observation and measurement of plaster models of the animal's jaws. The results were summarized as follows. 1. Measurement of the dental arch revealed that reduction in width rather than in length of the dental arch of the monkey was far greater as compared to the human dental arch, so that the general contour of both upper and lower jaws assumed an extremely lengthy shape, clearly distinct from that of human dental arch. 2. General contour of the palate of the monkey also showed an extremely small value of width as compared to its length and assumed an oblong shape as contrasted to the semi-oval shape of the human palate. 3. Width and thickness were measured on the crown part of the whole teeth. No appreciable sex difference was noticed in this regard except that the canine tooth crowns of upper and lower jaws in the male animal showed an extremely larger size than comparable crowns in the female animal. 4. When comparison was made among individual teeth group of the whole dentition on their relative sizes, the central incisor was larger than the lateral incisor among incisor of both jaws, and among the molar groups the smallest was the first molar while the second and the 3rd were markedly large sized. Although marked difference was not noticed among the premolar group, the 2nd premolar slightly excelled the Ist in size. These traits in tooth size, especially in the molar teeth group, were most characteristic of the monkey and differed considerably from the human counterparts where the largest molar came in the front as Ist molar, followed distally by the 2nd and 3rd molars each decreasing size in order. This relation was reversed in the molar group of the crab-eating monkey where the largest tooth was either the 2nd or the 3rd molar. The reason of this may be explained by a primitive stage of evolution still maintained in the animal and a retrogressive stage of the human molar teeth now going on. 5. The cusp contact relation between upper and lower molar cuspal surfaces was observed and classified as follows, based on the patterns in which cuspal grooves ran. In upper jaw of the animal, the most frequent type (I type) in the Ist molar was the one in which Paracone and Hypocone came in contact, in the 2nd molar the most frequent type was the one in which the four cusps converged to make contact at one middle point of the cuspal surface (II type), and in the 3rd molar the most frequent was the one in which Protocone and Metacone made contact with each other (III type). The last type found in the crab-eating monkey is distinct from the standard type seen in the molar teeth of mankind and ape where Protocone and Metacone are connected by an oblique ridge. In the lower jaw, the most frequent type of cusp contact relation was I type where Protoconid and Entoconid made contact and this was regarded as standard type in the crabeating monkey. In mankind and ape, however, III type made their standard type where Metaconid and Hypoconid came in contact and II and I types are regarded ordinarily as representing retrogressive trend. The most frequent type of cusp contact found in lower jaw of this animal, however, could not be designated as retrogressive in ordinary sense, but should be regarded as a specialized form of cuspal surface of the molar occurring exclusively in the genus Macaca. 6. Carabelli's tubercle was seen to occur in considerable frequency in the teeth of the animal, the majority of which,

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