IJf THE LOWER TEKTIAllY SXUAIA OP KKW ZEALAND. 181 



from the present sea-bottom- Even in these recent spicules 

 there is often an enhu'gement of the axial cauals due to a partial 

 solution of the spicular walls, and similarly enlarged canals are 

 likewise shown in these fossil forms. Not infrequently the 

 enlarged canals in the fossil forms have been infilled with a 

 greenish mineral which now appears as a slender axial rod dis- 

 tinct from the spicular wall, and in some cases this rod has sub- 

 sequently been contracted and contorted within the spicule so 

 as to resemble a foreign vermiform body. In many instances 

 also the Oamaru spicules have suffered from the peculiar borings 

 in their walls so common in spicules from the deep-sea deposits *. 



Not only have these Oamaru sponge-spicules retained their 

 original structure of opalized silica, but their forms have to a 

 great extent been preserved intact, with their surface adornment 

 of spines, &c., to the minutest microscopic detail. They have 

 naturally suffered less from mechanical pressure than the more 

 delicate diatoms and radiolaria whose broken up fragments 

 mainl}^ form the finer portions of the rock. 



In describing the spouge-remains in this deposit we are 

 necessarily limited to the characters shown by the detached 

 .spicular elements of these organisms, which are now indiscrim- 

 inately mingled together in the rock. Notwithstanding the 

 great abundance of these detached spicules, and the fact that they 

 belong to a great variety of sponges, no entire specimen of a 

 fossil sponge, nor even a connected fragment, appears as yet to 

 have been discovered in these beds. It is fairly certain that the 

 sponges lived and died at considerable ocean depths, and thus 

 were not likely to be exposed to any great disturbing infiuences 

 from currents ; and yet their skeletons seem to have been 

 thoroughly disintegrated, so that it is rare to find even two or 

 three of their spicules still in their natural association with 

 respect to each other. Not only is this the case with monac- 

 tinellid and tetractinellid sponges, whose spicules are merely held 

 in position by the soft animal structures, but it is equally true 

 with the spicules of lithistid sponges, and still more strange 

 with the connected meshw^ork of hexactiuellids, which occurs 

 broken up into microscopic fragments. There is consequently no 

 clue to the form or canal-structure of the sponges to which these 

 spicules belonged, and the only comparison available is that of 

 the relative similarity of the individual spicules with those of 



* See Duncan, Jouni. R. Micr. Soc. ser. 2, vol. i. (1881), p. 557, pis. vii., viii. 



