248 W. J. SOLLAS ON PHARETROSPONGIA STRAHANI. 



stance has in places penetrated the fibre and stained the exterior of 

 the spicules of the same colour as itself, thus rendering them more 

 than usually distinct on the unaffected transparent ground of crys- 

 talline calcite. 



The exceptional preservation of a group of spicules (PI. XI. fig. 8) 

 so stained at the point of anastomosis of three or four fibres in the 

 section PI. XI. fig. 5, gives an excellent opportunity for drawing 

 and measuring the individual spicules, some of which are represented 

 in PI. XI. fig. 9, magnified 435 diameters. These spicules are all ace- 

 rate, very sharply pointed, straight or curved, from 0*007 to 0*009 * of 

 an inch long, and about 0-00035 of an inch in diameter ; this latter 

 measurement has been checked by a comparison with the diameters 

 of the circles which represent transverse sections of the spicules, and 

 with concordant results. 



Prom the preceding it will appear that the calcareous fibres of this 

 sponge do not consist simply of homogeneous carbonate of lime, as 

 has hitherto been supposed, but are composed of a vast number of 

 acerate spicules, overlapping at their ends and lying parallel with 

 each other in the direction of the fibre. 



Prom this character we assign Pharetrospongia (pharetra, a 

 sheaf of arrows) as the generic name of the sponge; its specific 

 name, JStrahani, is in honour of Mr. Strahan, of H.M. Geological 

 Survey, who showed me a similar structure in an undetermined 

 sponge from the Upper Greensand of the south-west of England, 

 some years ago, before I had commenced the study of this so called 

 Chenendopora. 



Affinities. — The only sponges of the present day whose skeleton 

 consists wholly of acerate spicules are siliceous ones and belong to 

 the Holorhaphidota, an order established by Mr. Carter, and defined 

 by him as follows * : — " Possessing a skeleton whose fibre is almost 

 entirely composed of proper spicules bound together by a minimum 

 of sarcode. Porm of spicule variable." 



This order is divided by Mr. Carter into five families, of which 

 the first or Renierida is distinguished from the rest by having its 

 " spicules more or less arranged in a fibrous form," the very structure 

 which we described as characteristic of our sponge ; so far then we 

 have no difficulty, and Pharetrospongia must evidently be referred 

 to the Renierida. 



"When we come to its nearer relations we find a wonderfully closo 

 agreement between it and an unnamed sponge from Australia, 

 which Mr. Carter showed me for comparison, and of which, with 

 his usual generosity, he gave me notes and a specimen. The notes 

 are to the effect that the sponge is " massive, lobulate, with whitish 



fibre crammed with stoutish acerate spicules, g^ long by ^ Q0 

 broad, with very little sarcode." The specimen I imbedded in par- 



* It will be seen in the supplement that numerous isolated spicules have 

 since been exposed to view by a new process. These have for a maximum 

 length as much as from 0'01 to 0011 of an inch. 



t Ann. & Mag. Nat. Hist. ser. 4, vol. xvi. 1875, p. 130. 



