of the so-called Malm or Firestone Rock, 



V 



The simplest forms are straight or shghtly curved rods, 

 thickest in the centre, and gradually tapering to both ends. 

 Throughout the length of each spicule and opening at either end 

 is a slender axial canal, now filled with solid material, but of a 

 different aspect to that of the enclosing walls ; not mfrequently 

 this in-filling is of a greenish tint, and has been recognised as 

 glauconite, i. e., a silicate of iron and potash. These spicules, 

 with a simple unbranched axial canal, are termed monactinellids. 



In another form the spiciiles have each four pointed rays 

 springing fi-om a common centre in different planes ; each ray 

 has its axial canal, which connects ua the centre of the spicule 

 with those of adjoining rays. In some instances the rays are 

 subequal, but in others one of the rays is considerably elongated, 

 so as to form a shaft, from the distal ends of which the other 

 smaller rays diverge. These four-rayed spicules belong to the 

 group of tetractinellid sponges. 



In other spicules the form is very irregular ; the rays curve 

 and give oft' slight branches, which expand at the ends, and are 

 thus adapted for clasping each other. These spicules belong to 

 the group of lithistid sponges. 



And in yet another distinctive form each spicule has six rays 

 disposed at right angles to each other ; these are characteristic 

 of the group of hexactinellid sponges. 



Thus in this Malm rock are found detached skeletal spicules 

 representing each of the four principal groups into which existing 

 and fossil sponges are divided. 



It is somewhat remarkable, however, that though, as we have 

 seen, this rock is literally composed of the spicules of sihceous 

 sponges, there is not a single entire example of a fossil sponge to 

 be met with in it ; only the detached, isolated spicules. This 

 arises from the fact that in nearly all the sponges to which these 

 spicules belong the individual elements of the skeleton are not 

 organically united together in the living sponge, but are merely 

 held in their natural positions by the soft fleshy or merely 

 horny tissues of the animal ; and these, after the death of the 

 organism, rapidly decay, and the mineral particles or spicules, 

 losing their attachments, fall apart, and are scattered and indis- 

 criminately mingled together over the sea-bottom. In all 

 probability, generation after generation of sponges flourished in 

 the same area of the sea-bottom ; the scattered spicules of each 

 generation serving as a floor to support that immediately fol- 

 lowing, until by the gradual accumulation of millions of these 

 microscopic particles solid masses of rock have been formed ; 

 portions of which are now used for building houses and walls, 

 and for other purposes. 



Very few other organic remains than those of sponges can be 

 recognised in the Mahn rock of the district under consideration. 



