814 W. J. SOLLAS ON THE STRUCTURE AND 



anatomical structure so well preserved that we can trace it in nearly 

 all its details, even to distinguishing the peculiar apophyses which 

 terminate the ramifications of its spicules. So completely, indeed, 

 has the original form of the spicule been preserved, that the hemi- 

 spherical pittings, which generally excavate the walls of these struc- 

 tures in recent sponges after death, are to be observed in some cases 

 on the surface of these calcareous pseudomorphs. Thus the fact 

 that anatomical structure, and not mineral composition, should 

 guide us in our investigations into the characters of fossil sponges, 

 is hero enforced afresh, upon evidence scarcely less striking than 

 that which presented itself in the case of Stauronema Carteri*, a 

 fossil sponge belonging to the order Hexactinellidae. It will be 

 noticed also that we have here another case of the conversion of 

 colloidal silica into a crystalline state, so crystalline as to give 

 colours with Kicol's prisms quite as brilliant as those of mineral 

 quartz. Sometimes the spicules of the network are replaced by 

 iron-pyrites, so that, seen by reflected light, they glitter like bur- 

 nished brass. 



Near the exterior of the fossil the spicules have sometimes wholly 

 disappeared, and their places remain unoccupied as hollow casts ; or 

 more often these casts have become filled up with transparent 

 " coprolite," which almost obliterates them, and is only prevented 

 from quite doing so by the presence of a quantity of fine granular 

 material along with it, which often collects into lines along the 

 sides of the cast. Occasionally, also, an infiltration of glauconite 

 accompanies the coprolitic infilling, so as to make it appear that the 

 spicule has been replaced by that mineral, which, however, is not 

 the case. 



2. Siliceous Specimens from Haldon. — The internal canals in these 

 remain to a great extent unoccupied, as mere hollow tubes, though 

 here and there they have become partly filled in with crystalline silica : 

 and very generally they contain a singular thread of silica, having 

 very much the appearance of a thick bristle passed in to show the 

 course of the canals ; it lies in the axis of the canal containing it, 

 and quite free from the walls, though sometimes it leaves this 

 position and becomes attached to the side of the canal. The sub- 

 stance of the thread gives colours with polarized light ; its exterior 

 has a chalcedonic appearance : and it accommodates itself accurately 

 to the course of its canal. From its irregular form, its position, 

 and the entire absence of any such structure from the phosphatic 

 specimens of the Gault, and in any known sponge, recent or fossil, 

 I should say decidedly that it is not a spicule or any other structure 

 proper to the sponge, but a subsequent formation produced during 

 the mineralization of the particular specimens in which it occurs. 

 Again, the position of the threads is sufficient to show that they 

 cannot be of stalactitic formation ; the running or dropping of water 

 could never take place so as to form a thin axial thread extending 

 throughout the length of a narrow, branching, and tortuous canal. 

 On the other hand the connexion between the canals and their 

 * Ann. & Mag. Nat. Hist. ser. 4, vol. xix. p. 1. 



