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



striction. Basal ring smaller than the sa,£;ittal ring, thorny, kidney-shaped, with four different gates ; 

 the two cardinal pores much larger than the two jugular pores. (The four basal pores are often 

 much larger than in the specimen figured by Blitschli. Also the number, form, and size of the 

 spines is very variable.) 



BimeTisions. — Height of the frontal ring 007 to 0'09, breadth 01 v to 0-2. 



Habitat. — Fossil in Barbados. 



Subgenus 2. Tristephaniscus, Haeckel, 1881, Prodromus, p. 445. 

 Definition. — Sagittal and frontal ring of equal size and form. 



4. Tristephanium quadricorne, n. sp. (PI. 93, fig. 7). 



Sagittal and frontal ring of equal size and form, larger than the circular basal ring. From the 

 four corners, in which the latter crosses the two former, arise four strong, divergent spines, branched 

 like a deer's antler. Some smaller spines are scattered on the rings, and a bunch of four spines arises 

 on the apical pole. The four upper gates are triangular, the four lower nearly semicircular, the 

 latter of equal size, half as large as the former. 



Dimensions. — Height of the frontal ring 0'13, breadth 0'14. 



Habitat. — South Atlantic, Station 342, depth 1445 fathoms. 



Genus 422. Tricyclidium} Haeckel, 1881, Prodromus, p. 446. 



Definition. — C o r o n i d a with eight large, partly fenestrated gates ; the four 

 upper or lateral gates larger than the four lower or basal gates. Skeleton composed 

 of three latticed complete rings, perpendicular to one another. 



The genus Tricyclidium differs from the preceding Tristephanium, its ancestral form, 

 in the development of loose rudimentary lattice-work along the rings, and therefore 

 bears to it the same relation that Plectocoronis does to Eucoronis. It may pass 

 directly over into Dictyospyris. 



1. Tricyclidium dictyospjyris, n. sp. (PL 93, fig. 13). 



Sagittal ring ovate, twice as thick as the two other larger rings, which become very thin, thread- 

 like at the lateral junction, and are both slightly viohn-shaped, in the middle sagittal plane con- 

 stricted ; the frontal ring larger than the basal. All three rings bear small scattered spines, which 

 are irregularly branched, and by anastomoses of the thread-like branches form small in-egular meshes 

 along the rings. The four basal gates are of equal size. 



Dimensions. — Height of the frontal ring O'll, breadth 0'15. 



Habitat. — Equatorial Atlantic, Station 347, depth 2250 fathoms. 



1 Tricyclidium = Composed of three small crossed circles; tjiV, xuKhHiov. 



