ホンドタヌキ (Nyctereutes Procyonoides Viverrinus Temminck) の歯の比較形態学的研究

Abstract

The teeth were measured and the crowns were studied morphologically with use of 101 skulls of racoon dogs (♂ : 51 and ♀ : 50). Racoon dogs belong to Canidae of Carnivora and their teeth resemble closely to those of dogs morphologically. The results were as follows : 1. No definite difference by sex in size was observed in any tooth. 2. On the whole, the measurements of height, width and thickness of each tooth showed the followings : Of the incisors, I_3 was the largest in both the maxilla and mandible, followed by I_2 and I_1, with the tooth closer to the mesial side being smaller. Of the molars, P_4 in the maxilla and M_1 in the mandible, which are carnassial teeth, were the largest. A way from these two teeth toward the mesial and distal sides, the teeth became smaller successively. However, the maxillary corwn thickness was the largest in M_1 and became smaller in M_2, P_4, P_3, P_2, and P_1 in order. 3. On the lingual side of the incisors, cingulum was found well-developed and tubercles were occasionally found on the mesial and distal sides. In the maxilla, development of the tubercles was definite in I_1 and I_2 but indefinite in I_3. Especially, the distal tubercles were not recognized in I_3. In the mandible, development of the mesial tubercles were quite poor. The distal tubercles were most developed in I_3. 4. Lingual crown supernumerary tubercles are said to appear frequently in the maxillary P_3 and mandibular P_4 in dogs. In racoon dogs, however, they were barely recognized in the maxillary P_3 and were not at all recognized in the mandibular P_4. 5. The maxillary P_4 is what is called a carnassial tooth, and is quite large and complex. The crown is compressed bucco-lingually, and is long and narrow mesiodistally. It is composed of three main cusps (protocone, paracone, and metacone) and supplemental tubercles such as metaconus. 6. The maxillary distomolars have four cusps, paracone, metacone, protocone, and hypocone. The protocone is the highest. In the ratio of the size of each cusp to paracone, M_2 is larger than M_1 in every dimension, which suggests poor growth of paracone in M_2. 7. On the basis of the ratio of talon area to occlusal surface area of the maxillary distomolars, talon in M_2 decreased as compared with M_1 and was considered to be on a regressive trend. 8. In the mandibular distomolars, M_1 is what is called a carnassial tooth. It is long and narrow mesio-distally and has 5 shap and pointed cusps, protoconid, metaconid, paraconid, hypoconid, and entoconid, which rise prominently. In M_2, paraconid was regressing. In M_3, paraconid had disappeared and entoconid, hypoconid, and metaconid in order are in the process of regression. 9. In the mandibular distomolars, growth of talonid as compared with trigonid was better than that in dogs. M_1 in particular showed an index over 100, reflecting quite good growth. 10. The ratio of the size of other teeth to the carnassial tooth was the largest in the cuspid and the two neighboring teeth to the carnassial tooth in both the maxilla and mandible.