3g DlOTYOSPONOID^ 



peripheral frill; a form anticipated in the Tiiysanodictya of the npper 

 Devonian, in which evidence of the prism-faces is still retained. In the greater 

 nnml)er of all the other known species represented by the genera Cleodictya, 

 Lkbedictya, Calathospongia and Lykodictya, the form is some modification 

 of a rapidly expanding obcone or vase, like the ultimate expanded apertural 

 portions of the Devonian nodose sponges. With all these generic modifica- 

 tions so unlike those prevailing in the Devonian, are still to be found a few 

 representatives of the genera Pkismodictya and Dictyospongia. 



Though the future \n\l doubtless afford much evidence helping to estab. 

 lish the mutual relations of form among the Dictyosponges, the evidence of the 

 present clearly points to the simple, slender, smooth obcone as primitive, 

 probably derived from the antecedent Plectodekmatid^ and Protospongid^, 

 and attaining its maxinmm in the upper Devonian ; to the prismatic form as a 

 primary derivation therefrom, a group also attaining its highest development 

 in the upper Devonian ; to the nodose and annulated forms as a secondary 

 derivation, and the explanate, non-prismatic vase-shaped species with their 

 various modifications, such as prevail in the lower Carboniferous faunas, as a 

 still later derivation from the fundamental type. 



Modes of Presekvation. 



The skeletal substance of the siliceous sponges is a hydrated or chalcedonic 

 silica. In these fossils, as preserved in the older rocks, this skeleton has been 

 wholly removed and replaced by some other substance.* In a single instance 

 among all the examples here described, Clathrospangia Hamiltoneiisis, from the 

 calcareous shales of the Hamilton group, the skeleton has been substituted by 

 a deposit of carbonate of lime, which retains the general aspect of the net- work 

 though the form and structure of the individual spicular bodies is wholly lost. 

 In all other cases that have come under our observation the replacement of 

 the siliceous skeleton has been a complicated one resulting in the substitution 

 of iron pyrites for the silica, and with such delicacy and so gradually has this 

 process been carried forward, that the minutest details of form in spicules of 

 highly intricate structure have been retained. 



NotAvithstanding the fact that, almost without exception, in all the 

 numerous sjionges of the upper Devonian sandstones, the spicular skeleton is 

 entirely gone, it is evident that its absence is not due to a lack of replacement 

 ^y pyrites but to a removal of the pyrites itself. This appears from the 



•ZiTTEL observes that in the older lithistid siliceong sponges the orisinal amorphous, sintrly refractive 

 silica has become doubly refractive, and almost iusoluble in caustic alkalies {AnnaU and Magazine oj 

 Natural Hitlory, vol. 20, 1877, pp. 260 et seq.). 



