AFFINITIES OF THE GENUS SIPHONIA. 811 



With regard to their arrangement, it will be observed that while 

 previously (pp. 806-810) we described the cylinders of network about 

 the radiate canals from upper, lower, and lateral longitudinal 

 aspects, we are here viewing them transversely or end on ; and thus 

 no arrangement of spicules in series appears. What especially 

 strikes one is : — first, the uniform length of the radiating arms, which 

 here lie more or less parallel with the plane of the surface, while 

 the spicular shafts pass inwards at right angles to the surface ; and 

 next the fact that the arms diverge from one another more fre- 

 quently at angles of 120° than in the interior lateral views, so that 

 they are generally equiangular. 



Sections taken from these specimens may be examined as opaque 

 objects. Observations so made simply confirm results previously 

 obtained. 



Specimens of S. costata from the Wiltshire Greensand. — Of this 

 species I possess only one specimen (PI. XXV. fig. 2) ; and it, in com- 

 mon with all others I have examined, has been infiltrated with silica to 

 a much greater extent than the preceding specimens; and, as a con- 

 sequence of this, the skeletal network is much better supported, so 

 that Mr. Cuttell has been successful in obtaining a tolerably thin 

 slice from it. In this section the structure has in many places been 

 obliteratedlby mineral changes ; but enough remains distinct to show 

 that it possesses in all essential respects the same characters as the 

 species we have already discussed. 



Specimens of S. (Choanites) Konigii from the Chalk of Sussex. — 

 These have suffered greatly from mineral changes, the precise nature 

 of which will be described in a subsequent paragraph (p. 817). 

 Notwithstanding this, however, enough of the skeletal structure 

 remains to determine its real character. The quadriradiate spicules, 

 of the same form and size as in the other species, are united into a 

 similar network ; and there can be no doubt as to the Lithistid 

 character of the sponge. The tubercular extremities are unfor- 

 tunately not preserved so well ; all one can say of them is that they 

 were at all events confined to the ends of the spicular rays, and that 

 what traces they have left behind accord best with an origin in 

 tubercles of the same kind as those of other Sij)honice. After a 

 careful search through a beautiful series of Choanites, I have no 

 hesitation at all in referring them to the genus with which they are 

 here associated. 



Structure of the Stem of Haldon$vpkom.zd (PI. XXYI. fig. 7). — The 

 spicules which are exposed on the exterior of the stem of the 

 Haldon specimens differ in an interesting way from those of the 

 body, owing probably to physiological adaptation. The arms of 

 these spicules are greatly elongated, and bent in a direction parallel 

 with the long axis of the stem, and therefore with each other ; or, 

 at all events, those arms which do not take this direction remain 

 short, whilst the produced ones always lie longitudinally. Again, 

 all the spicular rays, whether bifurcated or not, appear, as far as 

 one can see in unprepared specimens, to terminate in simple pointed 

 extremities without forming clusters of botryoidal apophyses ; hence 



