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



branched thorns and distorted edges. Jugular pores triangular, scarcely half as broad as the large 

 semicircular cardinal pores. 



Dimensions. — ^Height of the sagittal ring Oil, breadth 0'08. 



Habitat. — Fossil in Barbados. 



7. Semantrum signarium, n. sp. (PI. 92, fig. 4). 



Sagittal ring obliquely ovate, thorny. Basal ring kidney-shaped or nearly pentagonal, with 

 broad concave dorsal incision and five larger branched spines on the five corners ; Ijetween them 

 smaller thorns. Jugular gates pear-shaped, scarcely half as broad as the triangular cardinal gates. 



Dimensions. — Height of the sagittal ring 0'12, breadth 0'09. 



Habitat. — Central Pacific, Station 271, depth 2425 fathoms. 



Genus 409. Semantidium, n. gen. 



Definition. — S e m a n t i d a with six basal pores (two anterior jugular, two middle 

 cardinal, and two posterior cervical pores), without typical basal feet. 



The genus Semantidiuin exhibits a further developmental stage of the basal plate of 

 the Semantida. Whilst this plate in Semcintis possesses one pair of basal pores, and 

 in Semantrum two pairs, here in Semantidiuin it has three pairs. The new third 

 pair is produced on the dorsal edge of the basal plate, which becomes connected with 

 the basal part of the dorsal rod of the sagittal ring by a pair of cervical rods. The 

 middle pair of pores (the cardinal) are always larger than the anterior (jugular) and 

 the posterior (cervical pores). The same form of basal plate is preserved in numerous 

 Spyroidea and Cy r t o i de a, as a " cortinar septum with six collar pores," (e.g., 

 PI. 53, fig. 18). 



1. Semantidium hexastoma, n. sp. (PL 92, fig. 6). 



Sagittal ring ovate, thorny. Basal ring rhombic or nearly square, with four short conical 

 descending spines on the four prominent edges (two sagittal and two lateral) ; between them 

 numerous smaller ii-regular thorns. Jugular and cervical gates nearly equal, ovate, half as broad 

 as the triangular cardinal gates between them. 



Dimensions. — Height of the sagittal ring 014, breadth 01. 



Habitat. — Central Pacific, Station 265, depth 2900 fathoms. 



2. Semantidium sexangulum, n. sp. 



Sagittal ring ovate, with six pairs of short branched thorns (two apical, two dorsal, and two 

 ventral). Basal ring hexagonal, with three pairs of short iDranched thorns on the six corners (one 



' >S'em(Mi{trfMim= Small signet-ring; a^imuTihiou. 



