CALCAREOUS SPONGES. 733 



Yon Lendenfeld [1891] vised the genus Leucetta to include 

 " Leuconidfe with triacts or tetracts or both." 



Dendy [1893 A] included Haeckel's species of Leucetta in the 

 genus Leucandra, on the ground that the mere absence of quadri- 

 radiate or oxeote spicules, or both, could not be regarded as of 

 generic significance. 



As a result of further study of the Calcarea, we are convinced 

 that the genus Leucandra, as used hitherto by Dendy, is not a 

 natural one, but must be subdivided, although on lines diflerent 

 from any that have previously been suggested. It appeai-s to us 

 that certain species, namely those which we now assign to 

 Leucetta, have originated, quite independently of the remainder, 

 from the homocoel sponges through a leucascid ancestry, and 

 have consequently never passed through a Sycon stage in their 

 phylogeny. The species in question are characterised by their 

 equiangular triradiates, and by the absence of any trace of the 

 typical Sycon skeleton. 



As regards the canal system, it appears that the same course of 

 evolution, from a condition with elongated and more or less 

 radially arranged flagellate chambers, to a condition with small 

 scattered spherical chambers, has taken place in both cases, so 

 that there has been a very complete convergence between the 

 genera Leucetta and Leucandra as now understood by us. The 

 true Leucandras, however, are distinguished by more or less 

 distinct traces of the skeletal structure exhibited by their 

 sy conoid ancestors. 



This view, so far as our present information goes, is supported 

 by histological evidence ; for, as already indicated, the position 

 of the nucleus of the collared cells in Leucetta is basal, while in 

 typical Leucandras it appears to be apical. 



We also include in the genus Leucetta, as now conceived, 

 certain species which we have hitherto regarded as belonging 

 to the genus Leucilla. These species are characterised by the 

 development to a varying extent of an inwardly pointing apical 

 ray on some of the trii-adiates of the dermal cortex. They are to 

 be distinguished from the true Leucillas on the same grounds as 

 those on which the other Leucettas are distinguished from the 

 true Leucandras, and we do not consider the mere presence of 

 such apical rays by itself as a suflicient justification for generic 

 separation. 



As we are taking Haeckel's type species of Leucetta as the 

 type of the genus as understood by us, we consider that we are 

 justified in retaining his name for the genus, although that name 

 has been used in different senses by subsequent writers. 



We allocate the following species to this genus : — 

 Section A. Without oxea. 



1. L. CHAGOSENSis Dendy. 



Leucetta chagosensis Dendy [1913]. 



