990 



THE VOYAGE OF H.M.S. CHALLENGER 



(the two rings being of different sizes), in the latter parallel (the two rings being equal) ; 

 the fundamental form of the former is a truncated pyramid, of the latter a prism. 



Comparing these differences between the four subfamilies of Tympanida, we may 

 suppose that the common ancestral group are the Protympanida, derived from the 

 Semantida by the formation of a mitral ring. If the two horizontal rings become closed 

 by lattice-work, the Paratympanida arise. When the mitral ring only becomes latticed, 

 the basal ring being simple, the Dystympanida originate. Finally, the fourth subfamily, 

 the Eutympanida, may be derived from the Protympanida by the loss of the two 

 horizontal parts of the sagittal ring. 



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 C y r t o i d e a. 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 (PI. 93, figs. 16, 22), 

 one odd caudal foot and two paired pectoral feet. Sometimes a fourth (sternal) foot is 

 developed (PI. 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) bi- 

 sected by the complete 

 sagittal ring. 



Two horizontal rings con- / One complete sagittal ring 

 nected by two columellae. \ (no frontal ring), 



Two hori- 

 zontal rings 

 connected 

 by four 

 columellse 

 (the halves of 

 the sagittal 



and the 

 frontal ring). 



No equa- 

 torial rinse. 



An equa- 

 torial ring. 



No galear and thoracal 

 bows, 



Galear and thoracal bows, . 

 Equatorial ring complete, 

 Equatorial ring incomplete, 



II. Subfamily 



Paratympanida. 

 Two horizontal rings 

 fenestrated. 



Two horizontal rings con- \ Six cohimell* (three rneri- 

 nected by six or eight 1 dlonal rm S 8 )' ' 

 columellae (three or four 1 -^. , , , ,, 

 vertical rings). El S ht ****& (fonr men- 



[_ dional rings), 



Two horizontal rings closed 

 J by a lattice-plate, con- J 



Two .F^ unequal (cola- 

 mellse dl rgent), 



nected by 

 columellfe. 



numerous 



I Two rings equal (columelhe 

 [ parallel), 



425. Protympanium. 



426. Acrocubus. 



427. Toxarium. 



428. Microculms. 



429. Odotympanum. 



430. Tympaniscus. 



431. Tympanidium. 



432. Paratympanum. 



433. Litlwtympanum. 



