An implantable device for use in fusing a pair of vertebrae together comprising: (a) a bowtie-shaped interspinous spacer body including a narrow central portion having a transverse dimension and having a pair of lateral ends each having a parallel, second, transverse dimension, larger than said first transverse dimension, the body including: (i) a first, cranially-facing, vertebra-engaging face; (ii) a second, caudally-facing, vertebra-engaging face, said first and second vertebra-engaging faces being generally concave and facing oppositely apart from each other and extending from the central portion towards the lateral ends; (iii) a ventral side having a ventrally-oriented face adjacent the first vertebra-engaging face, said ventrally-oriented face including a recessed portion shaped to bear against a dorsal aspect of a vertebral surface, and a lower portion protruding a distance in a ventral direction relative to said recessed portion and being of a size to fit within a cavity defined between inferior articulating processes of a vertebra after hemilaminotomy; (iv) a dorsal face; (v) a pair of opposite lateral end faces (82,8 4); and (vi) a pair of fastener receptacles (88, 90), each said fastener receptacle extending from said dorsal face through said spacer body diagonally to a respective one of said opposite lateral end faces; and (b) a pair of fasteners each engaged in a respective one of said receptacles and extending laterally outwardly at an acute angle with respect to said dorsal face, and in a ventrally inclined direction from respective one of said opposite lateral end faces (82, 84) far enough to engage and immobilize a respective facet joint between a pair of vertebrae. (a) a bowtie-shaped interspinous spacer body including a narrow central portion having a transverse dimension and having a pair of lateral ends each having a parallel, second, transverse dimension, larger than said first transverse dimension, the body including: (i) a first, cranially-facing, vertebra-engaging face; (ii) a second, caudally-facing, vertebra-engaging face, said first and second vertebra-engaging faces being generally concave and facing oppositely apart from each other and extending from the central portion towards the lateral ends; (iii) a ventral side having a ventrally-oriented face adjacent the first vertebra-engaging face, said ventrally-oriented face including a recessed portion shaped to bear against a dorsal aspect of a vertebral surface, and a lower portion protruding a distance in a ventral direction relative to said recessed portion and being of a size to fit within a cavity defined between inferior articulating processes of a vertebra after hemilaminotomy; (iv) a dorsal face; (v) a pair of opposite lateral end faces (82,8 4); and (vi) a pair of fastener receptacles (88, 90), each said fastener receptacle extending from said dorsal face through said spacer body diagonally to a respective one of said opposite lateral end faces; and (i) a first, cranially-facing, vertebra-engaging face; (ii) a second, caudally-facing, vertebra-engaging face, said first and second vertebra-engaging faces being generally concave and facing oppositely apart from each other and extending from the central portion towards the lateral ends; (iii) a ventral side having a ventrally-oriented face adjacent the first vertebra-engaging face, said ventrally-oriented face including a recessed portion shaped to bear against a dorsal aspect of a vertebral surface, and a lower portion protruding a distance in a ventral direction relative to said recessed portion and being of a size to fit within a cavity defined between inferior articulating processes of a vertebra after hemilaminotomy; (iv) a dorsal face; (v) a pair of opposite lateral end faces (82,8 4); and (vi) a pair of fastener receptacles (88, 90), each said fastener receptacle extending from said dorsal face through said spacer body diagonally to a respective one of said opposite lateral end faces; and (b) a pair of fasteners each engaged in a respective one of said receptacles and extending laterally outwardly at an acute angle with respect to said dorsal face, and in a ventrally inclined direction from respective one of said opposite lateral end faces (82, 84) far enough to engage and immobilize a respective facet joint between a pair of vertebrae.