REPORT ON THE TETRACTINELLIDA.. 
275 
Genus 1. Thrombus.} n. gen. 
With the characters of the family. 
Thrombus challengeri, Sollas (PI. VIII. figs. 24-39). 
Thrombus challengeri, Sollas, Prelim. Account, Sci. Proc. Roy. Dubl. Soc., vol. v. p. 179, 1886. 
.? Gortieium Mttoni, Carter, Ann. and Mag. Nat. Hist., ser. 4, voL xiv. p. 24, 1874. 
Sponge (PI. VIII. fig. 24). — An amorphous fleshy mass, incrusting a species of 
Rhizochaliim, and containing, embedded in its substance, numerous little spiral shells. 
Oscules and pores not seen. Surface irregular, the embedded shells everywhere pro- 
truding from it, smooth between the shells ; viewed with a lens finely granulated by 
the projection of the cladi of the underlying trichotrisenes. 
Spicules. — The only spicules present are trisenes, of which there are several varieties, 
all modifications of one prevailing form. 
1. Plagiotrisene (PI. VIII. figs. 25, 26), rhabdome straight, almost cylindrical, 
esactine tapering but slightly to a strongylate termination ; ecactine dividing into three 
simple cladi, which are strongylate and but slightly tapering, curved somewhat sigmately, 
and directed at first more forwards than outwards, and finally more outwards than 
forwards. The spicule is minutely spined all over, except near the origin of the actines 
and cladi ; the spines are erect, conical, sharply pointed, and spreading at the base 
tent-like. Ehabdome OT by O'OllS mm., cladi 0’055 by O'OllS mm.; one or two cladi 
of this spicule are occasionally absent (PI. VIII. fig. 29). 
2. Dichotrisene (PL VIII. figs. 27, 28), one, two, or all three cladi may bifurcate ; 
the dichotrisenes are usually smaller than the plagiotrisenes. Ehabdome 0'075 by 
O'OllS mm., protocladi 0'0197, deuterocladi 0'0158 mm. long. 
3. Trichotrisene (PI. VIII. fig. 30), one, two, or all three cladi may be trifurcate, the 
middle deuterocladus continuing the direction of the protocladus. When only one or 
two of the cladi are trifurcate, the remainder may be either simple or bifurcate, or one 
simple and the other bifurcate. Sometimes one or two cladi are suppressed (PI. VIII. 
fig. 31). Ehabdome 0'055 mm. in length, the proto- and middle deuterocladus together 
0'0355 mm. in length, the deuterocladi 0'0158 mm. 
The spicules are scattered isolately and without order in the choanosome ; in the 
ectosome, however, they are directed with the rhabdome pointing away from the surface 
at right angles to it, and the cladi extending tangentially beneath it. The cladome is thus, 
as is the case with all true trisenes, distally situated with respect to the actinal centre. 
Frequently the cladome may be seen projecting beyond the outer surface of the sponge, 
as though the spicule were in process of being extruded. 
Colour. — A somewhat dark slate-grey. 
^ o', a lump or clot. 
