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



The family T y m p a n i d a (or Parastej)liida) differs from the other S t e p h o i d e a. 

 in the development of two horizontal rings, which lie in horizontal planes and arise from 

 the upper and the lower poles of the primary sagittal ring. Therefore the same formation, 

 which we found in the Semantida on the basal pole only of the main axis, here takes 

 place also on its apical pole. On both poles arise at each side from the sagittal ring two 

 corresponding pairs of curved branches : the lateral ends of the opposite branches (dorsal 

 and ventral branch) become united in the frontal plane and so form two com}>lete 

 horizontal rings. The upper ring, composed of the superior or mitral branches, may be 

 called the mitral ring, the lower ring, formed by the inferior or basal branches, the basal 

 ring. Both rings are commonly of nearly equal size, more rarely of different sizes. Their 

 connection is effected either by the primary (sagittal) ring only, or also by secondary 

 vertical rings, a frontal ring iu the lateral plane, or some diagonal meridian rings 

 between the latter and the former. These vertical rings, which connect both horizontal 

 rings in the form of " columelli©," may be either complete or incomplete; in the latter 

 case their apical and basal parts are lost, the dorsal and ventral parts only being 

 preserved. 



The Tympanida develop a great variety of different forms, greater than in the three 

 other families of Stephoidea, so that we may distinguish here not less than sixteen 

 genera and seventy-two species. Hitherto only three species were known : Prismatium 

 tripleurum, figured in 1862 in my Monograph, TyiniKinidmm harbadense, figured by Bury 

 in the "fossil Polycystins of Barbados," and Tympaniscus Jibida, described in 1875 

 by Ehrenberg as a Ceratosjoyris Jihula. The number of species in this large and richly 

 developed group will be considerably augmented l)y further researches. A great many of 

 them exhibit remarkable relations to different Nassellaeia. For better survey of the 

 whole family we distinguish here four subfamilies, the first of which (Protympanida) is 

 probably the common ancestral group of the other three. They may have been derived 

 originally either from the Semantida or directly from the Stephanida. 



The first subfamily, Protympanida, exhibits a complete primary or vertical sagittal 

 ring, bearing on the upper part a horizontal mitral ring, on the lower part a horizontal 

 basal ring. Therefore the gates of these two parallel rings become bisected by the 

 sagittal ring, each divided into a right and a left half. The sagittal ring itself becomes 

 divided by the two horizontal rings into four parts or rods ; two opposite of these are 

 more or less vertical ; the dorsal (posterior) and the ventral (anterior) rod ; the two 

 other rods are more or less horizontal ; the mitral (upper) and the basal (lower) rod. 



In Protympanium (PI. 93, fig. 14), the simplest and most primitive form of 

 Tympanida, the connection of the two horizontal rings is effected only by the dorsal and 

 ventral rods of the sagittal ring forming two parallel more or less vertical " columellse." 

 In all other Protympanida accessory columellse are developed between the latter. In the 

 greater number we find two secondary columeUee in the lateral plane, being the lateral 



