This is a "Conical Bore" Mellophone made by E. A. Couturier  about 1930.  Couturier was known as one of the top cornetists in the US at the end of the 19th century.  He also composed music and ultimately designed and patented a new tubing system for brass instruments.

In 1916, Couturier, Melvin G. Lathrop, and William N. Barlow opened their own business, "E. A. Couturier Co., Ltd.," with the intention of producing their own line of conical bore instruments. Couturier transferred all of his patents from J. W. York & Sons to his new company. According to their catalogue of 1916, his company did produce an entire line of conical bore instruments, including a saxophone. In 1918, the firm moved from New York to LaPorte, Indiana, and was bought by Lyon & Healy in 1923. It operated until 1929. Please also see my other Couturier instruments: a Cornet, a Trumpet, a Baritone a Slide Trombone, and a Tuba.  Also please click here for a detailed bio of E.A Couturier.

This mellophone is pitched in Eb, and the respective valve crooks are fixed due to the conical design. The valves are slanted at about 45 degrees and are operated by the right hand.