malignant mesenchymoma
Malignant mesenchymoma has now virtually fallen into disuse, since the majority of sarcomas showing two or more lines of differentiation would nowadays be recognized, in most cases, as examples of tumours such as dedifferentiated liposarcoma or malignant peripheral nerve sheath tumour with heterologous elements.
There do exist exceedingly rare sarcomas showing two lines of differentiation which would not fit into the above categories, but these do not seem to form a clearly definable or discrete group.
References
The evolving classification of soft tissue tumours: an update based on the new WHO classification. Fletcher CD. Histopathology. 2006 Jan;48(1):3-12. PMID: #16359532#