Remarkable Pharetronid Sponge from Christmas Island. 125 
a layer on the surface of the sponge. All doubts as to Zittel’s discovery 
were now set at rest. Neither Zittel nor Hinde attempted to arrange the 
numerous genera of Pharetrones into sub-groups, but described the external 
form, the canal system, the size, shape, and structure of the fibres, and, 
wherever possible, the spicules composing them. 
The spicules which make up the so-called “ Pharetron fibre” are closely 
packed, and arranged more or less parallel to each other and to the long 
axis of the fibre. The spicules may be rod-shaped, or nearly linear three- 
rayed forms, or large centrally situated three- or four-rayed forms surrounded 
by slender rods or triradiates. 
The composition of the Pharetron fibre has been the subject of much 
discussion. Dunikowski (3, p. 299) and Rauff (7, p. 204) considered that the 
spicules were, in the living sponge, simply in apposition and not cemented 
together, and that the present fused vitreous condition of the. fibres has 
resulted from fossilisation. Steinmann (8, p. 111), on the other hand, 
considers that the spicules were joined together by a cement formed by the 
living sponge. Dunikowski regarded the Pharetrones simply as a sub- 
family of Haeckel’s family of Leucones. Rauff, who thought that Dunikowski 
had gone too far in so doing, retained the family Pharetronide, on account 
of the spicules being arranged into fibres, the spicules of the Leucones not 
being so arranged. 
In 1892 (1, p. 148) and 1898 (2, p. 15) Doderlein described a remarkable 
recent stony calcareous sponge (Petrostroma schulzer) from Japan. The main 
skeleton was constructed of large four-rayed spicules cemented together 
so as to form a firm solid meshwork, the spicules in the older parts being 
more or less completely surrounded with calcareous cement; also there 
was a dermal layer of loose three- and four-rayed spicules and separate 
small bundles or “ fibres” of “tuning-forks.” Doderlein placed Petrostroma 
in a new group, the Lithones. 
In 1893 Rauff (7, p. 204) classified all the Calcarea, fossil and recent, into 
two orders, Dialytina, with separate spicules, and Lithonina (including only 
Petrostroma schulzer, Dod.), with spicules joined by zygosis into a solid 
framework (like the desmas of Lithistida). It was not known at the time 
that the spicules of Petrostroma were joined together by cement, as was 
explained by Doderlein (2, p. 15) in his second communication on 
that genus. Rauff placed the Pharetronide next to the Leuconide. 
Whether Rauff’s division of Calcarea into two groups, viz., one with 
separate spicules and one with zygosed spicules, could have been retained or 
not, it is now needless to enquire ; but it could hardly be maintained that the 
Calearea should be divided into two orders, viz., one without cemented 
VOL. LXXXIII.—B. L 
