550 ME. JAMES JOHNSTOKE OK THE 



wards from about the transverse level of the first rib to about tbe 

 middle of the pericardial sac. Sometimes, however, as in the 

 young TricJiosurus referred to above, it lies quite above the first 

 rib. On the contrary, in at least the case of a foetal Perameles 

 I have found it to extend down below the apex of the heart, 

 and I suspect, from the appearance of the fat-mass round the 

 heart in Dasyurus, that a similar disposition had obtained there. 

 This downward extension of the thymus to the heart apex in some 

 Marsupials is somewhat remarkable. As a rule it lies quite 

 ventral to the pericardial sac, but here too there is an exception 

 in at least Didelphys pusilla, where it extends dorsally and 

 laterally until it approaches dorsally the mid body -line. There is 

 thus some considerable variation both in size, shape, and extent 

 of degeneration at corresponding periods in the life-history. 



With the exception of some of the foetal forms I have examined, 

 my material was but ill adapted for close histological examination. 

 I have not observed Hassall's corpuscles to be invariably present, 

 and never in the early stages of development. The best examples 

 of these structures I have seen were in the degenerated thymus 

 of the adult TricJiosurus referred to above. 



The case of PJiascolarctus is a remarkable one. Alone among 

 the Marsupials I have examined, this form gave no evidence of 

 possessing a thymus. But the specimens were young ones, and 

 since it seems to be the case that in many Marsupials this organ 

 develops comparatively late in life, it would be easy to overlook 

 the very small rudiment of a thymus which might possibly be 

 present. But I am convinced that in the mediastinal cavity 

 there was no such organ, either in rudiment or vestige. Erom 

 analogy to other Phalangerid* and the Macropodidae, it 

 was in such a case that a cervical lobe of the organ might 

 reasonably be looked for. This, however, after diligent scrutiny 

 I failed to find. 



It may be worth while to give here a diagrammatic represent- 

 ation of the system of organs in Macropus Eugenii. The figure 

 has been reconstructed from a series of sections, but is not 

 accurately drawn to scale. 



The thyroid is present in the form of 3 rudiments or lobes, a 

 median unpaired (tli.) and two lateral and jD^ired lobes (th."). 

 The thymus is represented by four lobes in the thorax, two on 

 either side of the middle line. The anterior pair (tm.a.) difier 

 notably, as we have seen, from the posterior pair (tm.p.). These 



