Harvey B. Roll— On F(m'l Sponges. 71 



Tnis sand mid gravel, in which the.?e fossils are embedded, are loosely 

 cemented by carbonate of lime, and it is by no means -certain at what 

 period, after the sponges were entombed, this coating of crystals was 

 deposited upon them. But it is quite possible that something similar 

 to what has taken place in these Farringdon fossils may occasionally 

 occur in other cases as a preliminary to fossilization, in consequence of 

 the sponge, deprived of its sarcode, having long soaked in water 

 charged v.ith carbonate of lime. 



The ordinary mode in which fossilization takes place, however, is in 

 one of the two"^ following ways : either the sponge after the destruction 

 of its sarcode becomes infiltrated by fine sediment, which completely 

 fills up the interstices and forms, as it were, a mould of the sponge 

 skeleton, in which the fossilizing process takes place ; or, the sponge is 

 simply buried in the deposit, which forms a nidus about it, filhng per- 

 haps the tubules and oscular passages, or even the superficial parts of 

 the tissue, but leaving the latter for the most part open and pervious, 

 into which mineral matter is carried in solution and there deposited. 

 The result of this may be, either to fill up the interspaices entirely, or 

 merely to encrust and consolidate the fibers as in the sponges of the 

 gravels of Farringdon ; but in either case there is formed around the 

 fiber or twig, a mould, in which the fossilization process takes place, 

 which is the same precisely as that which is known to take place m fos- 

 sils generally, viz., the removal of the original material of the skeleton, 

 and its replacement by another. These changes are greater and more 

 complete in proportion to the antiquity of the deposit in which the 



fossil occurs. 



III. Pictet converted D'Orbigny's fiimilies into tribes, and intro- 

 duced some additional genera created by Giebel, King, etc. ; and, ex- 

 cept in the description of new genera and ' species by Reuss, Pvoemer, 

 Salter, Eichwald and others, the subject remained very much where 

 D'Orbigny left it, until M. De Fromentelle proposed a new arrange- 

 ment, based upon what he terms the "organs which serve for the nu- 

 trition of the sponge," viz., the tubule, oscules, pores, etc. Like D'Or- 

 bigny, he divides the sponges into two orders : 1st, the Spongiana, 

 which comprises only recent genera ; and 2d, the Spongitaria, which 

 contains all the fossil genera, with the exception of the doubtful group, 

 the Clionidaj. The second order is further divided into three sub-orders: 

 1, those sponges which have one or more tubules {the Spongitaria 

 tabalosa) ; 2, those that have oscules, but no tubule (Spongitaria os- 

 culata) ; and 3, those that have neither tubule nor oscules (Spongitaria 

 porosa). Each of these sub-orders is further divided, thus: the tubu- 

 lar sponges into those in which the tubule is solitary, and those iu 

 which it Is grouped, and also into those with oscules and those without 



