992 THE VOYAGE OF H.M.S. CHALLENGEE. 



2. Protympanium amphipodium, n. sp. (PI. 93, fig. 14). 



Mitral and apical rings of equal size, thin, elliptical, smooth, larger than the broad connecting 

 sagittal ring. The latter bears on the apical and on the basal pole two divergent straight spines 

 (lying in the frontal plane), the two apical (or horns) somewhat smaller and not so spiny as the 

 two basal spines (or feet). In the frontal plane there arise from the two horizontal rings on each 

 side two small opposite spines (remnants or beginnings of the lateral frontal ring ?). 



Dimerisions. — -Diameter of the two horizontal rings O'll, of the sagittal ring 0'07. 



Habitat. — Central Pacific, Station 27l, depth 2425 fathoms. 



3. Protympanium trissocircus, n. sp. 



Mitral ring smaller than the basal ring ; both rings circular, rough. Sagittal ring in size 

 between the two former. All three rings covered with very small thorns. 



Dimensions. — Diameter of the mitral ring 0"08, of the sagittal ring 01, of the basal ring 0-12. 

 Habitat. — Central Pacific, Station 268, depth 2900 fathoms. 



Genus 426. Acrocuhus,^ Haeckel, 1881, Prodromus, p. 447. 



Definition. — T y m p a n i d a with two bisected horizontal rings, connected by four 

 vertical columellse. No equatorial ring. 



The genus Acrocubus is the most primitive of those remarkable Tympanida, the 

 .skeleton of which represents more or less exactly the form of a geometrical cube. A 

 simple complete sagittal ring bears on the superior and on the inferior part two opposite 

 pairs of lateral branches ; by union of the convergent branches of each side there arise two 

 horizontal parallel quadrangular rings, which are bisected by the sagittal ring. There- 

 fore the superior ring encloses two triangular mitral gates, the inferior two triangular 

 basal gates. The lateral corners of the two gates of each side are connected by a 

 vertical rod or columella. These two parallel columellse are the remaining middle parts 

 of the secondary or frontal ring, which is incomplete on the superior and on the inferior 

 face of the body. If the sagittal ring also become incomplete, by the loss of the 

 superior and inferior part (the anterior and posterior only remaining), then Acrocubus is 

 .transformed into Lithocubus. 



Subgenus 1. Apocubus, Haeckel. 

 Definition. — Basal ring without descending feet. 



^ ^crocM6tw = Perfect culie ; axjo?, xi/,3of. 



