Subfamily 3. TETRASPYRIDA, Haeckel, 1881, Prodromus, p. 442. _Definition._--#Zygospyrida# tetrapoda, with four descending basal feet, two of which are opposite in the sagittal plane (an odd caudal and an odd sternal foot), whilst the two others are paired lateral or pectoral feet. Genus 451. _Tetraspyris_,[69] Haeckel, 1881, Prodromus, p. 442. _Definition._--#Zygospyrida# with four basal feet crossed in pairs (two sagittal and two lateral). Apex with a horn. The genus _Tetraspyris_ and the following closely allied _Tessarospyris_ represent together the small subfamily of Tetraspyrida, characterised by the possession of four descending basal feet, three of which correspond to the three original feet of the {1044}ancestral genera _Cortina_ and _Tripospyris_, whilst the fourth in an odd anterior or sternal foot, produced by anterior prolongation of the basilar segment of the sagittal ring. Here, therefore, two opposed feet lie in the sagittal plane (a caudal and a sternal foot), whilst the two others are the paired lateral or pectoral feet, as also in _Stephanium_ and _Stephaniscus_, p. 965. The Tetraspyrida ought not to be confounded with the Therospyrida (sixth subfamily), in which the four feet have another signification. Subgenus 1. _Tetrarrhabda_, Haeckel, 1881, p. 429. _Definition._--Feet simple, not branched nor forked. 1. _Tetraspyris stephanium_, n. sp. (Pl. 95, fig. 6). Shell nut-shaped, tuberculate, with deep sagittal stricture and small roundish pores; three pairs of larger pores on each side of the ring. Basal plate with four large collar pores. Apical horn stout conical, half as long as the shell. Two pectoral feet somewhat longer than the two sagittal feet, one and a half times as long as the shell. All four feet straight, three-sided prismatic, strongly divergent. _Dimensions._--Shell 0.08 long, 0.12 broad; horn 0.04 long, feet 0.08 to 0.12 long. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. 2. _Tetraspyris cubica_, n. sp. Shell nearly cubical, smooth, with slight sagittal stricture and small polygonal pores; some larger pores on both sides of the ring. Basal plate with four large collar pores. Apical horn and the four feet of equal size and form, about as long as the shell, cylindrical in the basal, spindle shaped in the distal half. _Dimensions._--Shell 0.09 diameter; horn and feet 0.1 long. _Habitat._--Central Pacific, Station 265, depth 2900 fathoms; also fossil in Barbados. Subgenus 2. _Tetracorethra_, Haeckel, 1831, Prodromus, p. 429. _Definition._--Feet branched or forked. 3. _Tetraspyris tetracorethra_, n. sp. (Pl. 53, figs. 19, 20). _Tetracorethra mirabilis_, Haeckel, 1881, Prodromus, p. 439, and Atlas, loc. cit.