The two horizontal rings (upper mitral and lower basal) and the two vertical rings (primary sagittal and secondary frontal), which thus compose the loose framework of the Tympanida, are rarely smooth, commonly armed with spines or branches, similar to those of the Coronida. The rods or bars are either roundish (with circular transverse section) or angular (commonly with triangular transverse section). The paired branches of the two vertical rings often preserve the characteristic arrangement inherited from the Semantida and Stephanida. The two horizontal rings sometimes exhibit typical apophyses, which recur in the #Spyroidea# and #Cyrtoidea#. The mitral ring may possess one odd apical horn and two paired frontal horns (Pl. 93, figs. 16, 17, 19, 23). The basal ring sometimes bears the three typical feet of _Cortina_ (Pl. 93, figs. 16, 22), one odd caudal foot and two paired pectoral feet. Sometimes a fourth (sternal) foot is developed (Pl. 94, fig. 4), at other times only two lateral feet (Pl. 94, figs. 5, 6). _Synopsis of the Genera of Tympanida._ ------------------------------------------------------------------------- I. Subfamily Protympanida. Two horizontal rings (upper mitral ring and lower basal ring) bisected by the complete sagittal ring. ------------------------------------------------------------------------- Two horizontal rings {One complete sagittal connected by two { ring (no frontal columellæ. { ring), 425. _Protympanium_. } {No galear and Two horizontal} { thoracal bows, 426. _Acrocubus_. rings }No equatorial { connected } ring. {Galear and by four } { thoracal bows, 427. _Toxarium_. columellæ } (the halves of} {Equatorial ring the sagittal }An equatorial { complete, 428. _Microcubus_. and the } ring. { frontal ring).} {Equatorial ring } { incomplete, 429. _Octotympanum_.