FoKM. 85 



even among forms belonging to the genus Dictyospongia, so that tins pris- 

 matic form seems in a certain sense normal to the group, though always a 

 progressed condition. In some of the annulated species, as Riiabuosiponcha, 

 the annulations are clearly of subsecpient appearance to the prism-faces, 

 though the latter are represented throughout the adult condition of the sponge 

 as vertical ridges; while in Ceuatodici'YA the annulations appear to follow 

 directly upon the smooth conical condition, the prismatic condition not being 

 manifested. 



It is a significant fact that even with our present impei-fect knowledge 

 of this group, the earliest known Dictyosponge, Dictijosjxyrujia Danhyi, 

 McCoy (sp.), from the Ludlow beds of England, is such a regularly obconical 

 species with, possibly, an obscure development of the prism-faces toward its 

 upper part. This simple form may l)e regarded as directly derived from that 

 which characterizes such earlier Silurian forms as Cyatiiouictya. The 

 secondary modifications of this form (prism-faces, nodes, annulations, etc.), are 

 not known in any such early species nor outside of the DicTYospoNdiD.E. 



Among the many prismatic or banana-shaped species which constitute the 

 genus Prismodictya, the early smooth conical stage is extremely abbrev- 

 iated, but when specimens are well preserved to the basal apex it is found 

 that such a condition exists. 



With the disappearance of the Devonian faunas, the simple prismatic 

 sponges quickly disappear. The introduction of the Waverly fauna, immed- 

 iately succeeding the upper Devonian in Pennsylvania and Ohio, brought in 

 8e\'eral new generic types, some of which retain the prismatic form under 

 considerable modification (Calathospongia), but in the preponderating 

 species the prismatic structure is essentially lost. Tiiamnodictya and Cleo- 

 DicTYA are sponges with a smooth surface ; the simple, obconical form of the 

 cup is modified in both, in the former the base of the cup being broad and its 

 apertural portion expanded into a %vide vase, and in the latter, the bowl-like 

 lower part of the sponge bearing a row of nodes which seem to have no rela- 

 tion to prismatic faces as in all of the nodose species of the Devonian. 



The still later fauna of the Keokuk group as represented in eastern 

 Indiana, brings in the remarkable genus Piiysospongia, a broadly conical sponge 

 with wide base, and having the inner wall of its skeleton constructed on a very 

 simple plan ; a modification of the normal type of structure which is evidently 

 foreshadowed in the Roemei'ispongia Gerohtehiensis, F. Roemer, of the middle 

 Devonian of Germany. In this fauna is, also, the genus Piiragmodictya, a non- 

 prismatic sponge characterized by its depressed conical base surrounded by a 



