250 W. J. SOLLAS ON PBARETROSPONGIA STRAHANI. 



Formation. — Coprolite-bed at the base of the Chalk-marl*. 



Locality. — Cambridgeshire. 



State of Fossilization. — The intermeshes of the network are filled 

 up with the material of the bed in which the fossil happens to occur ; 

 thus specimens from the Chalk have their network imbedded in 

 chalk, and from the Cambridge Greensand in the heterogeneous 

 mixture of calcareous silt, fragments of crystalline calcite, coccoliths, 

 " green grains," JForaminifera, entire and fragmentary, and other 

 debris which characterizes that stratum, and which may be best 

 defined as impure chalk-marl. This chalk-marl usually forms a 

 hard compact mass in the intermeshes of the network, more or less 

 opaque and white ; but sometimes it is coloured brown, owing to an 

 infusion of phosphatic matter, so that the fibres of the fossil stand 

 out as a white network traversing a brown ground. 



In the best-preserved portions of the fibre the spicules are con- 

 verted into clear and transparent calcite, while the spaces between 

 .them, at one time filled with animal matter, are now occupied by 

 the finely granular opaque white substance which we have before 

 mentioned. In a less perfect state of preservation the fibre consists 

 wholly of crystalline calcite, which often exhibits a radiate crystal- 

 line arrangement, diverging from various centres, either in the 

 midst of the fibre or from the sides of its walls. Between the com- 

 plete preservation and the complete obliteration of the spicular struc- 

 ture there exists a whole series of transitions : a fibre full of spicules 

 at one end may consist of mere crystalline calcite at the other ; and 

 -between the two extremes will be found a number of intermediate 

 stages produced by the gradual disappearance of the granular coat- 

 ings of the spicules as these become converted into the crystalline 

 form. 



The total replacement of the silica of the spicules, which has taken 

 place as well when the outline of the spicules is preserved as when 

 it is not, can be most readily demonstrated by dissolving specimens 

 of Pharetrospongia from the Chalk in hydrochloric acid, when, save 

 for a green grain or two and a little structureless flocculent material, 

 no trace of a residue is left behind. We have here renewed 

 evidence of that preservation of anatomical details in a structure 

 which has undergone a complete chemical change, which I have 

 before called attention to in the case of Stauronema (Sollas), the 

 only difference between the two cases being that in Pharetrospongia 

 the anatomical structure is more delicate, and its replacement total 

 instead of incomplete. 



The spicules are sometimes subject to another change not yet 

 noticed ; and that is the appearance in their interior of minute 

 structureless spherules of a clear brown colour, often deepening into 

 black, and of about the same diameter as the spicules themselves. 

 Sometimes there are but one or two such spherules in a single 

 spicule ; sometimes a continuous series occurs along a considerable 

 length of spicule (PL XI. fig. 13), looking like bullets filling a tube 



* The genus is common in the Chalk ; but whether the same species occurs 

 there I have not now the opportunity of determining. 



