1564 APPENDIX. 



surface, the arrangement of the plates at these points being peculiar. 

 The basal pole (the starting-point of growth) is constituted by a 

 circle of eight (or four) plates. The apical pole is closed by a 

 variable, but always large, number of plates. 



4. Each of the five arms of the skeletal elements or spicules is 

 traversed by an axial canal, the canals of the four tangential arms 

 having a conspicuously fusiform shape. 



5. The radial arms or pillars terminate on the inner or "gastral" 

 side in a conical dilatation, which is laterally extended till adjoining 

 pillars touch. This internal thickening of the radial pillars is not 

 furnished with a special plate, corresponding with the external plate, 

 and is not penetrated by transverse canals. 



6. The inner or " gastral " wall of the fossil is imperforate, the 

 pores described by Billings being the result of fossilisation. 



7. The genus Ischadites agrees essentially with Receptaculites in 

 structure, but its skeletal elements are more slender. An apical 

 aperture is in some cases clearly wanting in Ischadites, and probably 

 did not exist at all. 



8. The genus Acanthochonia is identical with Ischadites. 



9. The geological range of Ischadites extends to the Upper 

 Devonian. 



10. The genus Polygonosphcerites (Sphcerospongia) is similarly 

 constructed to Receptaculites as regards the tangential arms of the 

 spicules, but the radial arms or pillars are wanting. 



1 1 . The Receptaculitidcs are not siliceous organisms, but the skele- 

 ton was originally calcareous, and the siliceous examples are the re- 

 sult of silicification. The group, therefore, cannot be referred to 

 the Hexactinellid Sponges, and its systematic position is still entirely 

 uncertain. 



Mastopora and Cyclocrinus (p. 186). 



In the memoir just referred to, Herr Rauff expresses the opinion 

 that the genera Mastopora (JVidulites) and Cyclocrinus have no rela- 

 tionships with the Receptaculitidcs, but that they appear to be related 

 to the Polyzoa. 



Calcispongle (p. 178). 



Some interesting discoveries as to the occurrence of Mesozoic 

 Sponges have recently been made by Dr George J. Hinde, who has 

 been good enough to furnish the following note of his investigations 

 on this point : "In the Middle Lias of Northamptonshire (zone of 

 A7nmonites spinatus) some very minute Calcisponges have been 

 lately discovered, with the same delicate structural characters as in 

 the existing family of the leucones, Haeckel, and they have even 



