ATRACTOSELLA— PLECTODERMA. 123 



SILURIAN SPONGES. 



Sub order. — Monaotinellilve. 



Genus. — Atractosella, 1 Hinde, gen. nov. 



The form of the Sponge is unknown, the genus is proposed to include detached 

 spindle-shaped spicules, with blunted extremities. In some spicules the central 

 portion is thickest, and they gradually and evenly taper to both ends, but more 

 frequently the greatest thickness is nearer one end of the spicule than the other, 

 and thus it tapers abruptly to one extremity and gradually to the other. 



8. Atractosblla siluriensis, Hinde, sp. nov. Plate I, figs. 6, 6 a — 6 d. 



The spicules are circular in transverse section ; they vary from -82 mm. to 

 1'62 mm. in length, and from '12 to "25 in thickness ; judging from broken frag- 

 ments, even larger forms are also present. Their surfaces are smooth and even. 

 The spicules retain their siliceous structure, and in some the axial canal is shown. 

 As the spicules are entirely detached from each other, there are no means of 

 determining the nature of the Sponge to which they belong, but provisionally they 

 may be referred to the Monactinellidae. Spicules of this order are very rare at 

 this horizon, and no others have as yet been met with in this country. The forms 

 figured were discovered by Mr. John Smith, of Kilwinning, in decayed limestones. 



Distribution. — Silurian : Wenlock Beds, near Craven Arms, Shropshire. 



Sub-order. — Hexactinellid^;. 



Family. — Protospongil\e. 

 Genus. — Pleotoderma, Hinde. 



1883. Catalogue Fossil Sponges Brit. Mus., p. 132. 



Generic Characters. — Entire form of Sponge unknown ; the wall consists of a 

 thin spicular meshwork composed of cruciform and five-rayed spicules, and possibly 



1 drpaKTos, a spindle, dimin. 



