REPOET ON THE TETRACTINELLIDA. 
XCl 
silica of flint nodules, it by no means follows that other kinds of organisms have not 
assisted. Schulze notices the comparatively rich development of Hexactinellid Sponges 
in association with Diatomaceous ooze, and I have observed something similar in the case 
of the Tetractinellida, If the contribution of these organisms and of Eadiolaria be taken 
into account, the time required for the formation of the material of a bed of flints may be 
still further shortened. 
4. ONTOGENY. 
Eemarkably few instances of early developmental stages have been met with. In 
Craniella schmidtii (?) (p. 39, El. XLII. figs. 20, 21) and Craniella simillima (p. 33, 
PI. II. figs. 18, 19 ; PI. XL. fig. 5) planulse and young sponges were observed within the 
parent. Very young sponges, evidently having only just completed their embryological 
development, were observed in the case of Thenea schmidtii (p. 69, PI. VIII. fig. 22) 
and Stelletta phrissens (p. 152, PI. XVI. figs. 15-20). In sponges not in the Challenger 
collection I have observed similar very young sponges in the case of Draginastra 
normani and Geodia barrettii ; the young examples of the latter furnish valuable evidence 
bearing on the mode of development of the cortex ; this will be found recorded in the 
^revision of the family Geodiidee. Unfortunately no examples of young sponges were 
furnished by the Lithistida. 
External Gemmation. — The only Tetractinellid genus in which external buds have 
been observed is Thenea; Vosmaer was the first to describe them in the case of Thenea 
muricata^ they also occur in Thenea schmidtii (p. 69, PI. VIII. fig. 21). So long as 
the buds remain attached to the parent they do not present any structure by which 
their true nature can be recognised, and this, therefore, at present is a matter of 
inference, resting partly on analogy with the structure of the similar buds of Tethya. 
As the external buds of Tethya are not further generally alluded to in the Appendix on 
the Monaxonida a few words must be devoted to them here. They were first discovered 
by Mr. T. H. Stewart, and described by Bowerbank in Tethya lyncurium ; ^ in the same 
sponge they were subsequently investigated by Dezso,® and in an allied species, Tethya 
maza (p. 440), by Selenka;^ in another also closely allied species, Tethya seychellensis, 
they are described by Perceval Wright ; ® similar buds have been described by 
Merejkowsky in the Suberite, Rinalda arctica.^ In the Challenger collection they were 
met with in a new species, Tethya japonica (p. 430, PI. XLIV. figs. 11-13), as well as 
in Tethya seychellensis (p. 427, PI. XLIV. fig. 1), in neither of these instances was any 
1 Bijdragen tot de Dierhunde, vol. xii. p. 6, 1885. 
^ Mon. Brit. Spong., vol. i. p. 149, 1864. 
2 Archivf. mikrosk. Anat., Bd. xvi. p. 626, pis. xxx.-xxxiii., 1879. 
^ Zeitsehr. f. wiss. Zool., Bd. xxxiii., 1879. 
^ Trans. Roy. Irish Acad., vol. xxviii., Science, No. 2, 1881. 
® Mem. Acad. Imp. d. Sci. St. Petersb., ser. 7, tom. xxvi. No. 7, p. 5, 1878. 
