CALCAREOUS SPONGES. 803 



constitute the outstanding feature of the Heteropiidse. They 

 are, so to speak, a new discovery, which the sponge utilizes to 

 the utmost, until finally their strong centripetally directed oral 

 rays, in association with the opposed basal rays of subgastral 

 sagittal triradiates, give rise to an " inarticulate " tubar skeleton, 

 which replaces the articulate tubar skeleton of the ancestral Sycon. 

 The evolution of the canal system within the family appears 

 to have followed the usual lines up to a certain point. The 

 known species of Vosmaeropsis exhibit the type of canal system 

 described by von Lendenfeld as " sylleibid," intermediate between 

 syconoid and leuconoid, but a Heteropiid with a typical leuconoid 

 canal system has not yet been found. 



In the Grantiidee, on the other hand, the development of a 

 dermal cortex appears to have been inaugurated by the 

 appearance of a thin poi-e-bearing dermal membrane over the 

 ends of the inhalant canals in some syconoid ancestor, such as 

 is known to occur in some species of the genus Sycon at the 

 present day (e. g. S. boomerang). 



The Grantiidse must be regarded as the dominant family of 

 Calcarea at the present day, comprising, as they do, no less than 

 23 genera, and exhibiting a veiy wide range of structure both 

 as regards skeleton and canal system. It might be possible to 

 divide these genera into syconoid and leuconoid subfamilies, but 

 we should have no guarantee of the monophyletic origin of the 

 latter from the former. Nevertheless, the scarcity of sylleibid 

 forms, which might be regarded as connecting links between the 

 two types, is somewhat remarkable, and suggestive of a natural 

 cleavage. The known species of the genus Megapogon, iiowever, 

 are sylleibid, and althovigh most of the Leucandras have small 

 chambers, L. austrcdiensis Carter and L. infesta sp. n. {=.Leucilla 

 intermedia Row [1909]) have very large ones, and might also be 

 considered as transitional forms. Also we must remember that 

 in a considei'able number of cases we have no accurate information 

 as to the canal system. 



The ai-rangement of the genera within the family is a very 

 difiicult problem. They appear to group themselves around two 

 central types, Grantia and Leucandra, but as we have just 

 mentioned, our knowledge of the exact type of canal system in 

 many forms is very imperfect, while in others the only evidence 

 of their close relationship to Leucandra is the canal system itself. 

 The appeaiance of this part of our phylogenetic tree will pi'o- 

 bably be greatly modified by subsequent investigations. Such 

 aberi"ant genera as Leucopsila, Baeria, Kuarrliaphis^ Leucyssa 

 and Trichogypsia can only be included in the Grantiidas pro- 

 visionally. 



The difiiculty of arranging the genera probably arises from the 

 fact that great gaps exist in the family owing to extinction 

 of intermediate forms. It might be argued that this family 



Proc. ZooL. Soc— 1913, Is^o. LIII. 53 



