936 THE VOYAGE OF H.M.S. CHALLENGER. 



are complete, but the frontal ring is incomplete, its basal part being wanting (PL 82, figs. 

 4—6). Therefore we find here six large gates between the three rings; four upper lateral 

 gates (between the two crossed vertical rings) and two lower basal gates (between the 

 basal rod of the sagittal ring and the two halves of the basal ring). Eucoronis, the type 

 of this subfamily, may be derived either directly from Seviantis by development of a 

 frontal ring, or from Tristephanium by loss of the basal part of the frontal ring. 



A quite simple basal ring, with a single gate, distinguishes the fourth subfamily of 

 Coronida, the Acanthodesmida (sensu restricto), the genera Coronidium (PI. 82, 

 figs. 1, 2, 7, 8) and Acanthodesmia (PI. 93, fig. 5). The horizontal basal ring alone 

 is here complete, whilst both vertical rings (the sagittal and frontal rings) are incomplete, 

 their basal parts being wanting. Therefore there are here five gates, four lateral and one 

 basal. These forms may be derived from Eucoronis by loss of the basal rod of the 

 primary sagittal ring. 



The Tympanida, the fourth family of S t e p h o i d e a, exhibit another type of ring 

 structure. Here two parallel horizontal rings are constantly developed, one on the 

 apical pole, the other on the basal pole of the sagittal ring. The latter is the same 

 basal ring as in the Semantida and Coronida. The former is a " mitral ring," developed 

 in the same manner, by union of two paii's of horizontal lateral branches, which arise 

 on both sides from the apical rod (or mitral rod) of the sagittal ring. In the simplest 

 case these two parallel horizontal rings are connected only by the sagittal ring, which 

 is either complete (Pi'ott/mjxinium, PL 93, fig. 14) or incomplete (Parastephamis, 

 PL 93, fig. 21). But commonly also an incomplete frontal ring is present, so that 

 the two horizontal rings are connected by four vertical or subvertical rods ; two of 

 these " columellse " are the dorsal and ventral rods of the sagittal ring, the two others 

 are the lateral rods of the frontal ring. Between the former and the latter are 

 sometimes developed two, four, or more accessoiy columellse (probably halves of 

 accessory incomplete diagonal meridian rings). In this way arise the characteristic 

 " drum-forms " of many Tympanida, in which the two parallel horizontal rings 

 correspond to the upper and lower rings of a drum, whilst the connecting vertical 

 columellse correspond to its parallel lateral rods (PL 83, figs. 1, 2, &c.). 



Originally the two horizontal rings of these " drum-shells " are both bisected by 

 the complete sagittal ring, each provided with two lateral gates (Protympanida). But 

 in the Eutympanida both i-ings exhibit one simple gate only, the apical and the basal 

 rod of the sagittal ring being lost. In the Paratympanida both rings are closed by a 

 secondary lattice -plate, whilst in the Dystympanida the upper (mitral) ring alone is 

 closed by such a plate, the lower (basal) ring is open. In some Eutympanida the 

 shell assumes the strange form of a regular geometrical cube, the twelve edges of which 

 are represented by thin rods of silex [Lithocuhus) — its four upper edges represent 

 the mitral ring, the four lower the basal ring ; two opposite of the four vertical cube- 



