Spinal implants for positioning between adjacent vertebrae. Some embodiments may comprise a pair of opposed sidewall surfaces, a pair of opposed frictional surfaces, a first end wall joining the pair of opposed sidewall surfaces, and a second end wall recessed at least in part and joining the pair of opposed sidewall surfaces. A fish-tailed structure may be positioned within the second end wall when viewed from one of the opposed frictional surface. The fish-tailed structure may be formed so as to extend at least substantially uninterrupted between at least one of the opposed sidewall surfaces and the opposed frictional surfaces. A retention structure, such as a hole, may be positioned within the fish-tailed structure. The hole may be configured to engage a surgical implantation device.