PLATE LXXXIX. 315 



which this sponge might be confounded is I. fallax, 

 but in that species there are numerous tricurvate 

 tension spicula ; and the skeleton spicula are much 

 smaller and more delicate in their proportions, being 

 in their length as two to three in L incerta. 



The reticulations of their dermal membranes also 

 differ considerably. In I. fallax it is rarely ever more 

 than unispiculate, and very delicate in its general 

 aspect ; while in 7. incerta it is much coarser in its 

 structure. The difficulty in the discrimination of these 

 two species is also increased by their being as nearly 

 as possible of the same colour in the dried state ; but a 

 comparison of the size of the skeleton spicula of I. 

 fallax represented in Plate LI, fig. 11 with those of I. 

 incerta in Plate LXXXIX, fig. 4, will greatly assist 

 their discrimination ; the spicula of both species being 

 figured to the same scale. X 250 linear. 



TETHEA CRANIUM, Lamarck. 

 Plate LXXXIX. 



The propagation of the genus Tethea by gemmulation 

 exhibits some remarkable variations. One of the most 

 striking modes of external gemmulation is that which 

 obtains in^ Tethea lyncurium which I have described in 

 page 149, vol. i, of the present work, and represented 

 in Plate XXV, 342. In this case^the young sponge 

 elevated on its pedestal is constructed in a form closely 

 representing the adult state, the skeleton radiating 

 from its centre. In both cases described in vol. ii, p. 

 93 the gemmules pullulated from near the base of the 

 sponge, and no other mode of propagation was detected. 

 Since then I have found a very similar case of gem- 

 mulation in Tethea Ingalli from Australia which I have 

 described and figured in ' Proceedings of the Zoological 

 Society of London ' for 1872, page 115, plate v, fig. 12 ; 

 but differing in having the gemmule closely adherent 

 to the base of the sponge, without any indication of a 



