In all mammals described i n this report, the masseter consists of the proper masseter group which includes the first superficial, second superficial, intermediate and deep masseters and the improper masseter group which consists of the maxillo- and zygomaticomandibular muscles. These muscular elements can be identified exactly in every animal investigated. The special features are described as follows:
1) In the red kangaloo, the first superficial, second superficial and intermediate masseters are thin, though broad. The deep masseter group makes especially a good development, namely, the pars anterior is identified in the first and second layers and the pars posterior can be divided into the first-fifth layers. The improper masseter group which includes the maxillo- and zygomaticomandibular muscles makes a good development, which is comparable to those of the carnivorous animal.
2) In the primate, the t r ansitional change of the zygomaticomandibularis from the zygomatic arch to the supraorbital torus is pursued from the lower to the higher forms. In the galago, the muscle takes its origin from the zygomatic arch. In the squirrelmonkey, it originates not only from the zygomatic arch, but also spreads over the superficial temporalis, making the semicircular plate. In the chimpanzee, the muscle spreads between the superficial and deep temporales, connecting the lateral portion of the supraorbital torus with the coronoid process as in the human masseter.
3) The masseter of the guinea pig makes a complicated development. The first superficial masseter is divided into the pars anterior and pars posterior. The former corresponds to the pars reflexa (Tullberg), bifurcating into two branches. The maxillomandibular muscle consists of two parts as in the rat (Yoshikawaetal.,1961). One of them passes through the infraorbital foramen and the other originates chiefly from the zygomatic arch and terminates in the long sulcus in the mandible. The latter corresponds to the zygomaticomandibularis after Toldt (1905), Schumacher (1961, b) and Schumacher-Rehmer (1962). The real zygomaticomandibular muscle is vestigial as in the rat (Yoshikawa et al.,1961). It develops between the posterior part of the zygomatic arch and the root of the glenoid process.
4) In the bear, the maxillo- and zygomaticomandibular muscles make a specially good development. In the adult form, the former reflects under the second superficial masseter, taking the anterior position to the intermediate masseter. The pars posterior of the deep masseter makes the well developed pyramidal form with rich tendons, the lamination of which, however, cannot be followed.
5) In the Indian elephant, the development of the maxillomandibular muscle is remarkable as in that of the horse. The pars anterior of the deep masseter cannot be identified.