202 CALCAEEOUS SPONGES. 



Small cylindrical forms are 47 mm. in length by 15 in thickness, whilst large 

 expanded examples reach the dimensions of 200 mm. in length and the same in 

 width. 



The outer surface of the wall is generally uneven and composed of a layer of 

 reticulate fibres, finer than those of the interior ; the interspaces between them are 

 small, irregular in distribution, and either subangular or circular. The inner surface 

 of the wall is usually smoother than the outer, the fibres are thicker, and the inter- 

 spaces larger; in some cases definite circular apertures up to 1-.3 mm. in width are 

 present, but generally the apertures are smaller and irregular in form. No definite 

 canals can be distinguished in the walls, even where they attain their greatest 

 thickness. In some instances the fibres have a general disposition in the direction 

 of the length of the sponge, with transverse connections ; but, as a rule, no definite 

 arrangement is perceptible. 



The fibres are from IcI to -'ob mm. in thickness; they are composed of minute, 

 straight, or slightly curved, subcylindrical uniaxial spicules, closely arranged parallel 

 with each other in the direction of the lenijth of the fibre. The long-est of these 

 spicules which I have succeeded in measuring is T4 mm. and its thickness about 

 ■01 mm. 



This species occurs in the' Cambridge Green Sand, and it is also very abundant in 

 the Upper Chalk. The Chalk examples are for the most part larger and the walls are 

 thicker than in the Green Sand forms, but the differences do not appear to me to be 

 of specific value. Many of the examples from the Green Sand near Cambridge are 

 entirely calcareous, and the fibres completely dissolve when treated with acid ; in 

 others the fibres have a thin surface-pellicle of silica and the interior is calcareous. 

 Attached to the siliceous pellicle are entire and fragmentary spicules which have 

 been replaced by silica, the spicules of the interior of the fibre retaining their 

 calcareous composition. A similar replacement has also taken place in some of the 

 forms from the Upper Chalk. Those which are imbedded in the chalk itself have a 

 smooth, waxy aspect, and when treated with acid completely dissolve ; but not infre- 

 quently specimens are imbedded in flint, and in these cases the exterior portion of 

 the fibres becomes siliceous whilst the interior retains its calcareous condition. That 

 this partial silicification of the fibre is owing to its being surrounded by the flint is 

 clearly shown by the fact that some sponges are partly imbedded in flint and partly 

 in chalk, and only the portion in the flint has been replaced by a thin coating of 

 silica, that in the chalk being wholly calcareous. 



By Prof. Sollas and Mr. Carter these sponges are believed to have been originally 

 siliceous, and, from the form and disposition of the spicules in their fibres, to be 

 allied to an existing species of Beniera. My reasons for regarding them as originally 

 calcareous are based upon the fact that in their general characters and structure 

 they closely resemble other Pharetrones occurring in the same strata, which by 



