Only three genera, Sagitta, Spadella and Krohnia, are recognised, and the number of species is small.
The three genera are differentiated as follows: - Sagitta M.
Let r1 be the radius of the large end of each, ri that of the small end, r, that of the middle; and let Ii be the sagitta, measured perpendicular to the axes, of the arc by whose revolution each of the conoids is generated, or, in other words, the bulging of the conoids in the middle of their length.
While the stroke of A is ACa, extending to equal distances on either side of C, and equal to twice the chord of the arc Dd, the stroke of B is only equal to twice the sagitta; and thus A is guided through a comparatively long stroke by the sliding of B through a comparatively short stroke, and by rotatory motions at the joints C, D, B.
V.) Tegyd; laborem gave llafur; sagitta gave saeth; remus gave rhwyf.