EA PHIDONEMA .— PH AEETROSPONGI A . 201 



These canal-apertures are sometimes bridged over by the fibres. In other examples 

 a smooth, minutely porous dermal layer, consisting of very delicate, closely reticu- 

 lating fibres, extends to varying heights in the cup, and covers over the coarser fibres 

 and the canal-apertures. This dermal layer is very variously developed ; in some 

 specimens it is limited to the lower portion of the cup or is entirely absent, whilst 

 in others it extends, like a thin film, quite to the margin, so that the interior of the 

 cup is perfectly smooth. The wall itself is perforated by numerous straight or 

 slightly sinuous canals, which run generally at right angles, but occasionally are 

 oblique to the surface. 



The fibres vary from -1 to -3 mm. in width, and are composed of filiform spicules 

 similar to those of R. contortum. In some cases the dwarfed basal ray of the three- 

 rayed spicules can be clearly seen ; but, as a rule, this minute ray cannot be detected 

 in microscopic sections, and thus the spicules appear as if uniaxial. Very rarely is 

 a perfect spicule exposed in the section ; the entire length of an apparently complete 

 form is •2.3 mm. 



The presence of minute three-rayed spicules in this and other species of this genus 

 distinctly marks them off from specimens of Pharetrospongia, in which only uniaxial 

 spicules have hitherto been discovered. 



Distribution. Lower Green Sand : Farringdon, Berkshire. Very abundant. 



Genus PHARETROSPONGIA, Sollas, 1877. 



Professor Sollas has based this genus on the characters of the single species 

 P. Straham, which is principally distinguished by the uniaxial form of the spicules 

 composing its fibres. Prof. Zittcl proposed to extend Sollas's definition so as to 

 include in the genus a group of cup- or funnel-shaped sponges having a general 

 resemblance in their mode of growth to P. Stmhani. As, however, the spicular 

 structure of the fibres of some, if not all, of these sponges difi'ers from that of 

 P. Strahani, it seems to me desirable to limit the genus to sponges which can be 

 shown to possess a similar spicular structure to that of the type species. 



Pharetrospongia Strahani, Sollas. (Plate XXXVII. fig. 6 ; 

 Plate XXXVIII. fig, 1.) 



1877. Pharetrospongia Strahani, Sollas, Quart. Journ. Geol. Soc. vol. xxxiii. p. 212, t. 11. 



1878. Pharetrospomjm Strahani, Zitt. Studien, III Ab. p. 45. 



1879. Pharetrospongia Strahani, Carter, Ann. & Mag. Nat. Hist. ser. 5, vol. iv. p. 432. 

 1882. Pharetrospongia Strahani, Steinm. Neues Jahrb. Bd. 2, p. 129. 



Sponges having the form of variously convoluted plates, which sometimes anasto- 

 mose so as to become funnel-shaped, also occasionally subcylindrical. The walls are 

 from 7 to 13 mm. in thickness. The sponge does not appear to have been attached. 



2d 



