542 ME. JAMES JOHNSTONE ON THE 



sevenths of the lateral diameter. Lower down it formed two 

 thick paired lobes whicli extended posteriorly to the apex of the 

 heart. 



lY. Phalangeridjb. 



Trichosurus vulpecula. (PI. 37. fig. 2.) 



Symington has described the thoracic and cervical portions of 

 the thymus both in the adult and foetus of this species, and 

 I have little to add to his account. In n)y youngest specimen, 

 an animal in good condition and measuring 19 centim. in length, 

 the thoracic thymus {tm.) consisted of three large lobes, rather 

 above the base of the heart, and a small spindle-shaped body 

 situated on the ventral aspect of the right carotid. The two 

 lateral lobes of the thyroid were joined by a distinct isthmus 

 {is., PI. 37. fig. 2), which is geuerally absent from the thyroid of 

 adult marsupials. Both my other specimens were old, and in 

 them the thymus was reduced to a mere nodule of fatty tissue 

 situated on the origin of the carotids, containing scattered 

 groups of thymus-cells. I have also examined sections of the 

 neck and thorax of a foetus, and found both thoracic and cervical 

 portions of the thymus as described by Symington. 



Cuscus orientalis. 



The thoracic thymus (PI. 37. fig. 5, tm.) lay in my specimen 

 anterior to the first rib (I.) and on the roots of the great vessels 

 («.c.', a.c"). It consisted of a median mass, undivided into lobes, 

 and sending two prolongations along the ventral sides of the 

 carotids, each terminating in a fine point. I could not make 

 out the presence of a cervical portion, but it is probable that I 

 may have overlooked its existence in this form. 



JPJiascolarctus cinereus. 



I had two specimens of this species, each measuring about 

 20 centim. from the snout to the root of the tail, and therefore 

 rather young. In neither case could I find any thymus. In the 

 mediastinal cavity itself I could detect nothing which might be 

 interpreted as either a rudiment or a vestige of a thymus, and in 

 the neck there were no structures of like nature. 



JPetaurus hrevieeps. Pull-grown specimen. 



The thymus was represented by a mass of tissue lying about 



