CALCAREOUS SPONGES. 713 



maintained by Bidder and Minchin, is another and much more 

 difficult question to decide. The presence of regular equiangular 

 triradiates seems most certainly to be very characteristic of the 

 Leucascid-Leucaltid line of descent, and we have made use of it 

 as one of the distinguishing features of the members of those 

 groups. It is, however, extremely difficult in practice to distin- 

 guish between a sagittal spicule which owes its sagittal character 

 merely to the backward bending of the oral rays, and one which 

 is sagittal owing to a real inequality between the primitive 

 angles. 



There can be no question that a superficially sagittal condition 

 may be attained in different ways, and one of the most interesting- 

 results at which we have arrived in the preparation of the present 

 paper is that the so-called subdermal sagittal (pseudosagittal) 

 spicules of the Heteropiidte have a quite different oiigin from 

 the ordinary sagittal form, the basal ray not being homologous 

 in the two cases. 



With regard to Jenkin's [1908 B] supposed families Chiphoridse 

 and Staurorrhaphidse, we have come to the conclusion that these 

 are based upon purely imaginary distinctions. It Avill be re- 

 membered that Jenkin maintained that in these families a special 

 type of spicule, the " chiactine," constitutes the first (or only) 

 joint of the tubar skeleton. It seems highly improbable, from 

 purely a priori reasons, that this joint should be diffei-ently con- 

 stituted in different syconoid sponges. As a matter of fact, no 

 one, so far as we are aware, has demonstrated how it arises in 

 ordinary cases, such as Sycon or Grantia, but everybody has been 

 content to speak of it as being composed of subgastral sagittal 

 trii-adiates. It is, moreover, well known that these triradiates 

 may develop an apical ray, as they do in many species (e. g. 

 Sycon ensiferum, /Sycon verum and Grantia indica), and thus 

 become quadriradiates. Jenkin thought that he had demon- 

 strated that in certain cases the subgastral quadriradiates aiise 

 by rotation and re-orientation of the basal rays of gastral quadri- 

 radiates, owing to the pressure of the developing chambers, and 

 regarded this mode of origin as distinguishing his so-called 

 chiactines from other subgastral tri- and quadriradiates. As, 

 however, he made no attempt to show how the subgastral tri- 

 radiates and quadriradiates arise in other cases, this distinction 

 cannot be regarded as having any value ; and we ourselves are of 

 opinion, from our own observations, that the spicules which con- 

 stitute the first joint of the tubar skeleton pi-obably arise in the 

 same wa}'^ in all cases, although we are not disposed to accept 

 without further evidence the exact mode of origin described by 

 Jenkin. 



It is certain that, in some cases at any rate, the spicules at the 

 growing margin of the osculum have not yet assumed their defi- 

 nitive orientation, but exhibit a confused arrangement. Some of 

 them gradually become oriented as dermal cortical spicules, with 

 their three rays lying parallel to the surface, others as subgastral 



