ASTROSCLERA WILLEYANA, THE TYPE OF A NEW FAMILY OF SPONGES. 477 



In C ruffosa, from the Upper Greensand of Warminster, Hinde succeeded in obtain- 

 ing complete and isolated tri-radiate spicules (loc. cit., PI. XI. Fig, 2.5), and sections of 

 well-preserved specimens of this genus commonly show the ' fibre ' to be composed of 

 such spicules. 



In Zittel's figure of a specimen of C. gracilis, Mlinstr. from St Cassian, alluded 

 to above, no trace of sjjicules is to be seen, and the ' fibre ' has instead a well-marked 

 spherulitic structure. 



Now on the view of the pi-imary nature of the bodies with radiating lines, making 

 up the spherulitic structure, we are driven to suppose that among the sponges which 

 have been regarded as Pharetrones there are two groups, one with spicular skeletal 

 elements, the other with polyhedral skeletal elements and that both have developed iuto 

 simple and segmented forms. It was shown in the last section that in its larger 

 characters Astrosdeni does remarkably resemble such unsegmented forms of the 

 Pharetrones as Stellispongia. But that there should also be segmented forms with 

 polyhedral skeletal elements resembling the segmented Pharetrones would indeed be an 

 astonishing instance of parallel development. 



b. Many intermediate conditions are met with between a ' fibre ' composed of 

 spicular elements and one exhibiting the spherulitic structui-e. Thus the fossils 

 belonging to the genus Gorynella have, as we have seen, in many cases, a fibre com- 

 posed of spicules. In a section of a specimen of C. forarninosa from the Lower Green- 

 sand of Farringdon\ while the spicular elements are clearly seen in the centres of the 

 fibres, their peripheral parts present a fine crystalline striatiou perpendicular to the 

 surface, and there is no sign of spicules in these regions. 



Again, though the spherulitic structure is so clearly marked in the specimen of 

 Gorynella gracilis Mlinstr. above alluded to, in one of my specimens (from Schluderbach) 

 referred by Dr Hinde to this species, the central parts of the trabeculae are made up 

 of elongated bodies which may well be the remains of spicules, though here too there 

 is at their periphery a crystalline striation more or less perpendicular to the surface, 

 though the lines are in part disposed in groups with a fan-like radiating arrangement. 



Similarly, in Pharetrospongia Strahani Sollas, the ' fibre ' is often seen to contain 

 spicules lying, as in a quiver, in a direction parallel to the .surface, but in one of 

 Dr Hinde's sections of a specimen of this species, no trace of spicules is to be seen, 

 and instead, the 'fibre' presents lines .sometimes disposed perpendicularly to the surface, 

 sometimes extending in fan-like and frond-like systems, with definite boundaries between 

 thcni, recalling the patterns on a window-pane in frosty weather. 



If the spherulites with fibres radiating in all directions are regarded as primary 

 elements of the skeleton, what, it may be asked, is to be made of the imperfect 

 systems, and of these borders of fibres set nearly perpendicularly to the surface of 

 the trabeculae, which are of much more frequent occurrence ? 



c. Although, in its more marked form the spherulitic structure does remark- 

 ably resemble that seen in Astrusclera there is, I believe, this difl:erence ; that whereas 

 the radiating lines in the latter run straight from the centre to the circumference of 



> Kiudly lent to me by Di- Hinde. 



vv. IV. 6't 



