Castor Canadensis(ビーバー)の頭蓋骨, 歯列弓, 口蓋および歯の形態学的研究

Abstract

The skull, dental arch, palate and teeth of beavers were measured in a morphological study made on 15 specimens. Measurements were taken studied for correlations, in particular those related to head shape, size, and the ratio of increase in various specimens. The results were as follows : 1. In the skull form, head length was long and head height was low. A significant positive correlation was found between head length, head breadth, and head height. The proportion between the measurements were invariable irrespective of head length. 2. The mandibular form was comparatively wide. A significant positive correlation was found between the measurements of the mandibular. The correlation between head length and mandibular size was higher than that of the mandibular body, dental arch, and palate. A longer head length indicated a higher increase in mandibular breadth when compared with the increase in mandibular length. 3. The mandibular head appeared like a globular form. The breadth between the mandibular angles was comparatively greater than the breadth between the mandibular heads. The height of the condylar process was less than that of the coronoid process. The mandibular notch was comparatively deep. The mandibular ramus was comparatively wide. The mandibular angle process was broad and roundish with a wide base. The mandibular body was comparatively thick. The site of mental foramen was slightly lower than the center and most posterior part of diastema. 4. Both upper and lower dental arches were widest at the third molar part. The dental arch forms were comparatively wide on both sides. The correlation between skull and dental arch length was higher than that between skull and dental arch breadth. A longer head length indicated a higher increase in dental arch length as compared to the increase in dental arch breadth on both sides. 5. Palate form was narrow and quite shallow. The lowest correlation coefficients were found in the palate. 6. In the incisors form, crown length was long, labiolingual diameter of crown was comparatively thick, and the mesiodistal diameter of crown was comparatively wide. A significant positive correlation was found between the crown parts. In the upper incisor, the lowest correlation coefficients were found in crown length. The longer the head length was, the wider labiolingual diameter of crown and the thicker mesiodistal diameter of crown were. In the proportions between the crown parts of incisor form was invariable irrespective of the head length. 7. The occlusion surface of molars was square. At the upper molars, the crown was compressed bucco-lingually, while at the lower molars, the crown was compressed mesio-distally. In the upper molars, the thickest buccolingual diameter of crown and the widest mesiodistal diameter of crown were found in premolar ; however, in the lower molars, the thickest buccolingual diameter of crown was found in premolar and the widest mesiodistal diameter of crown was found in first molar. A significant positive correlation was found between the mesiodistal and buccolingual diameters of all the upper and lower molars. In the upper first and second molars, the longer the head length was, the narrower the buccolingual diameter was. In the lower second and third molars, the longer the mandibular length was, the narrower the mesiodistal diameter of crown was. In the other molars, the buccolingual diameter of crown and the mesiodistal diameter of crown were invariablle irrespective of head length and mandibular length. 8. The above forms of each part were well adapted to the food habit of and the environment for beavers.