MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 75 



ramus. Posteriorly it lies between the mylohyoid and the styloglossus 

 muscles and anteriorly it is between the mandible and the genioglossus. 

 It extends from the vicinity of the base of the angular process to the 

 symphysis. Its ducts were not sought but it is evidently independent of 

 the other large salivary glands. 



Thyroid. In a male specimen, the thyroid was found in two elongate 

 pisiform divisions (PL VI, Fig. i) lying on the lateral surface of the 

 trachea. The larger anterior one extended from the vicinity of the 

 thyroid cartilage caudad about 4 mm. The smaller lay just caudad 

 in the same longitudinal plane and was about one-third the size of its 

 fellow. In a female specimen, only one thyroid was detected. 



Thymus. All the specimens examined were fully adult and show only 

 traces of the thoracic thymus. No indication could be found of the 

 presence of the superficial cervical thymus regarded as characteristic 

 of the Diprotodontia by Symington (1898) and Johnstone (1901). Four 

 distinct fatty lobes lying ventrad of the carotids and cephalo-ventrad of 

 the base of the heart were noted in one specimen as probably represent- 

 ing the thymus. 



Mammae. There are four mammae, regularly arranged, two on 

 each side of the abdomen inside the thighs. This is the number present 

 in the majority of diprotodonts and less than the number usually found 

 in the polyprotodonts. In comparison with the Didelphudae, however, 

 the number is perhaps less important than the regularity of arrange- 

 ment, for members of that family are distinguished from all other mar- 

 supials by having the mammae present only in odd numbers and more 

 or less asymmetrically arranged. The number in diprotodonts never 

 exceeds four; in polyprotodonts it is usually more than six, although 

 Thylacinus, Myrmecobius and a few species of Phascologale have only 

 four, while the greatly specialized Notary ctes has but two. The following 

 are the number found in the principal marsupial genera : 



