Page:Scientific results HMS Challenger vol 18 part 2.djvu/59

Rh or elliptical, or more polygonal, and is connected with the basal rod of the sagittal ring by three pairs of radial apophyses, the anterior furcular, the middle coracal, and the posterior scapular rods.

A third important form of Semantida is Semantidium (Pl. 92, figs. 6, 7). Here we find three pairs of basal pores in the seal-plate; the third pair, newly formed, consists of the cervical gates (e), bounded in front by the scapular rods (d), behind by a fourth pair of basal apophyses, the cervical rods.

The basal apophyses of the sagittal ring are not only of great morphological importance, because they produce by their union three typical pairs of basal gates or "collar pores," but also because their prolongations often appear as typical basal feet. The distal prolongations of the coracal rods appear in Semantiscus (Pl. 92, figs. 16-18) as two pectoral feet, those of the scapular rods as two tergal feet, whilst the opposite prolongations of the basal rod of the sagittal ring appear as two "sagittal feet" (in front an anterior or sternal, and behind a posterior or caudal foot). In the typical Cortiniscus (Pl. 92, figs. 11-13) only three feet are developed; an odd caudal and two paired pectoral feet (compare above, p. 891). The typical basal ring of the Semantida, with its paired basal gates (Semantis), reappears in the majority of the Coronida, differing from the former in the development of a second vertical ring, which lies in the frontal plane (perpendicular to the sagittal ring), and which we therefore call the frontal ring. In only one small group of the Coronida the basal ring is absent, namely, in the Zygostephanida, and here the frontal ring appears in the simplest form, as a complete elliptical meridian ring, crossing the sagittal ring perpendicularly on the two poles of the main axis (Zygostephanus, Pl. 93, figs. 1-4), Four large lateral gates between the two rings remain open. This form may be derived directly from the Stephanida in the following way; from both poles of a simple sagittal ring there arise two opposite lateral apophyses, which in the frontal plane become curved one towards the other, and united in the poles of the transverse axis. The basal apophyses would be the coracal rods. But it is also possible that Zygostephanus was derived from Semantis by the loss of the furcular rods.

The three typical rings (or the "dimensive rings") of the appear in their most complete form in the subfamily Trissocyclida (Pl. 93, figs. 7, 13). Here all three rings are undivided and completely developed in the three dimensive planes, perpendicular one to another. Between them there remain eight large open gates; the four superior are the four "lateral gates" of Zygostephanus, the four inferior are the four basal gates of Semantrum. The four latter are originally much smaller than the four former; but in Trissocircus and Trissocyclus (Pl. 93, figs. 10-12) they reach the same size. Therefore all eight gates are here of equal form and similar size, and the basal ring, now a true equatorial ring, divides the two meridional rings into two equal halves.

In the Eucoronida, a third subfamily of Coronida, the sagittal and the basal rings