{Six columellæ Two horizontal rings { (three meridional connected by six or eight { rings), 430. _Tympaniscus_. columellæ (three or four { vertical rings). {Eight columellæ { (four meridional { rings), 431. _Tympanidium_. ------------------------------------------------------------------------- II. Subfamily Paratympanida. Two horizontal rings fenestrated. ------------------------------------------------------------------------- {Two rings unequal Two horizontal rings closed { (columellæ by a lattice-plate, { divergent), 432. _Paratympanum_. connected by numerous { columellæ. {Two rings equal { (columellæ { parallel), 433. _Lithotympanum_. III. Subfamily Dystympanida. Mitral ring fenestrated, basal ring simple. ------------------------------------------------------------------------ Two horizontal rings } Mitral ring closed by a connected by numerous } lattice-plate, basal (six to eight or more } ring simple, 434. _Dystympanium_. columellæ). } IV. Subfamily Eutympanida. Two horizontal rings (upper mitral ring and lower basal ring) simple. Apical and basal parts of the sagittal ring lost. ----- {Two columellæ (parts of Two horizontal rings { the sagittal ring), 435. _Parastephanus_. connected by two or { three columellæ (four {Three columellæ (parts of or five open gates). { the half sagittal and { the frontal ring), 436. _Prismatium_. { {Two rings { { unequal, 437. _Pseudocubus_. {Four { Two horizontal rings { columellæ {Two rings connected by four or { { equal, 438. _Lithocubus_. more columellæ (six or { eight or more open { {Two rings gates). {Six to eight { unequal, 439. _Circotympanum_. { or more { { columellæ. {Two rings { { equal, 440. _Eutympanium_. Subfamily 1. PROTYMPANIDA, Haeckel. _Definition._--#Tympanida# with two bisected horizontal rings, connected by the complete vertical sagittal ring. (The upper mitral ring and the lower basal ring become divided by the complete primary or sagittal ring into two horizontal symmetrical gates, two mitral gates on the apical and two basal gates on the basal pole.) Genus 425. _Protympanium_,[44] Haeckel, 1881, Prodromus, p. 447 (_sensu emend._). _Definition._--#Tympanida# with two bisected horizontal rings, connected by two vertical columellæ.