Simplify the following text for a Grade 7 native English-speaking person.
Original: Even after the opening , legally the NHM remained a department of the British Museum with the formal name British Museum ( Natural History ) , usually abbreviated in the scientific literature as B.M. ( N.H. ) or BMNH .
Simplified: The NHM was at first a department of the famous British Museum . When the present NHM building was opened in 1881 , it was then called the British Museum ( Natural History ) or BMNH .