248 BRITISH JURASSIC SPONGES. 
Family.—LEUCONES (Bowerbank), Haeckel. 
1872. Die Kalkschwimme, vol. ii, p. 113. 
Calcisponges, whose usually thick walls are traversed irregularly by sinuous, 
branching, often anastomosing canals. The spicules are irregularly distributed 
in the body of the sponge. 
Owing to the loose indefinite arrangement of the skeletal spicules in the 
sponges of this family they are, as might be supposed, extremely rare as fossils, 
and the forms from the Middle Lias, described below, are the only representatives 
known up to the present time in the fossil condition, if we except a few detached 
spicules occurring in the Upper Chalk and in Pliocene strata. 
The chief difference between the Leucones and the Pharetrones, to which all 
the other Calcisponges herein described belong, consists in the fact that the 
spicules in this latter family are arranged so as to form solid anastomosing fibres, 
whereas in the Leucones they are loosely scattered in the soft tissues. 
Dunikowski has, however, maintained (‘ Paleontographica,’ vol. xxix, 1883, 
p- 34) that the fibrous character of the Pharetrones-skeleton is a secondary 
structure due to the fossilisation, and he has consequently considered the group 
as only a sub-family of Leucones. The character of the spicules and of the canal 
system in these families is admittedly very similar, but the markedly distinct 
fibrous arrangement of the skeleton, which appears to me to be original and not 
secondary in its nature, justifies, in my opinion, the family distinction assigned 
by v. Zittel to the Pharetrones as distinct from Leucones. 
Genus.—Luvoanpra, Haeckel. 
1872. Die Kalkschwamme, vol. ii, p. 170. 
Calcisponges with branching canals, in which the skeleton consists of three- 
and four-rayed spicules as well as simple rod-like forms. 
56. Leucanpra WaxForpI, Hinde. Pl. XIX, figs. 8—8 a. 
1889. Levucanpka Watrorp1, Hinde. Ann. and Mag. Nat. Hist., s. 6, vol. iv, 
p. 352, pl. xvii, figs. 1—9. 
Sponges small, club-shaped, subcylindrical or compressed, slightly contracted 
at the base, which is attached to grains of sand or fragments of other organisms. 
