SPONGES. 215 



in the siliceous form*, though but little change may have oc- 

 curred in the mineral substance. As a rule the calcareous spi- 

 cules were minute and intimately united in the sponge fibre, so 

 that it required but a slight amount of alteration to obliterate 

 their form in the resulting fibre-crystalline mass of calcite. So 

 far as observed, the spicules are never recognizable in the palaeo- 

 zoic calcareous sponges, excepting where they are isolated or 

 project from the walls into the canal cavities. Some instances 

 have also been met with in which the calcareous skeleton has 

 been more or less completely replaced by silica. Several 

 species of Heterospongia, a new genus with affinities to Dystac- 

 tospongia and Strotospongia, and a general resemblance to 

 Jurassic species of Corynella, present examples of this condition . 

 These, as they occur at Cincinnati, Ohio, are always calcareous 

 but at several localities in central Kentucky, where most of the 

 fossils are also silicified, the same species are found having 

 their skeletons strongly charged with siliceous material. The 

 sponge fibre of such specimens is rough and preserves scarcely 

 a trace of the spicular structure though in some the course of 

 the canals and twisted walls can be distinguished with approxi- 

 mate certainty. Generally, however, this style of preservation 

 is anything but favorable. 



GEOLOGICAL DISTRIBUTION. 



As may be expected, sponges make their appearance in the 

 earliest fossiliferous rocks, and they are more or less numer- 

 ously represented in all the succeeding great divisions of the 

 geological scale to the present time. They attain the height of 

 their development in the Jurassic and Cretaceous periods, in 

 which rocks they form an important feature of the fauna. Their 

 importance is less marked in Palaeozoic and early Mesozoic 

 groups, though they played a less subordinate part in Palaeozoic 

 times than is generally supposed. The fact is, they have been 

 too much neglected by palaeontologists, perhaps, because the 

 specimens are rarely showy and in most cases appear as unsat- 

 isfactory, shapeless masses, little calculated to enthuse the col- 

 lector. But the true naturalist should see beauty in all of 

 Nature's handiwork, and I feel convinced, when once an interest 

 is aroused in the study of these obscure fossil remains, their im- 



