368 DISEASE IN WILD MAMMALS AND BIRDS 
Tumors. 
Neoplastic diseases of the bones have yet to be classi- 
fied to even^one 's satisfaction. All gradations of hyper- 
plasia of osteogenetic cells and fibres and of the marrow 
elements, from simple inflammation to true sarcoma, are 
recognized. As one reviews a large series of lesions, 
clearly defined types may be found, but there are transi- 
tion stages to which an exact name is difficult or 
impossible to apply. Nor does the pathological diagnosis 
always fit with the clinical course. Thus, for example, the 
giant-cell tumor of bones looks malignant, and is not, and 
its structure may be simulated in such diseases as fibrous 
osteitis and Paget 's disease. As we have seen in the dis- 
cussion of the latter of these two, abnoraialities of fibre 
and cell gro^^i:h simulate neoplasms very closely. In 
addition it might be mentioned here that actinomycosis 
may produce bony growths resembling sarcoma. When 
osteitis deformans affects the facial bones especially, it 
has been called leontiasis ossium, and it is then a more 
nodular, tumor-forming process, the enlargement con- 
sisting of fleshy masses occupying the whole bone, but 
especially the marrow cavity. Histologically the lesion is 
fibrocellular, frequently with numerous giant cells; 
accumulations of small sarcoma-like alveoli may be found. 
We have encountered four cases among common opos- 
sums {Didelphys virginiana) and one in an Isabelline 
Gazelle {Gazella isabella) and, because of the localization 
and fleshy consistency of the tumor, we have called them 
osteosarcomata for descriptive purpose, but not for 
classification among neoplasms (where they will not be 
fomid). Two of the opossums had osteoporosis and 
gelatinous marrow in the ribs. All these animals had been 
in the exhibition under a year, and as far as known are 
not related. Figure 47 shows the gross character, while 
the following is the description from one protocol. 
