anadidymus

du·plic·i·tas pos·te·ri·or

conjoined twins in which there is a single head and upper body and duplicated buttocks and lower limbs. See: conjoined twins.
See also: dipygus.

anadidymus

(an″ă-did′ĭ-mŭs) [ ana- + didymos]
A developmental abnormality in which the upper parts of the bodies of twins are fused, but the buttocks and legs are free.