Page:Scientific results HMS Challenger vol 18 part 2.djvu/202

1078 The number and disposition of the terminal feet around the basal plate do not vary so much in the Tholospyrida as in the Zygospyrida. We find here only one genus with three feet, Tholospyris (corresponding to Tripospyris); one genus with two lateral feet, Lophospyris (corresponding to Dipospyris); and two genera with numerous feet, the horned Sepalospyris and the hornless Tiarospyris (the former corresponding to Petalospyris, the latter to Gorgospyris). The fifth genus (Pylospyris) has no feet at all, and may be derived from Circospyris among the Zygospyrida.

The cephalis exhibits in the Tholospyrida the same typical structure which we have described above of all (compare p. 1017). The cupola or galea, arising from its upper face, seems to be a secondary production, perhaps caused by an apical growth of the central capsule, or in other cases by descending branches of the apical horn, which cover the apical face of the calymma with protecting network.

Definition.— with three basal feet and an apical horn.

The genus Tholospyris is probably the common ancestral form of the Tholospyrida, and has arisen from Tripospyris by development of a galea or cupola; this is composed of the apical horn, the lateral branches of which become connected with lateral spines arising from the coryphal face of the cephalis, and of lattice-work developed between the former and the latter. The three descending feet of the basal face are the typical cortinar feet (one odd caudal and two paired pectoral), the same as in the old ancestral genera Cortina, Plagoniscus, Tripospyris, &c.

Definition.—Basal feet simple, neither forked nor branched.