16 DicriYOsi'ONr.iDA. 



Form. Six)nges assume an endless variety of expressions of a sac- 

 shaped body. Aside from iiiherent specific traits, Avhich express themselves 

 most forcibly in such variations, there are numerous external conditions which 

 superinduce modifications in form, even within the limitations of a species. 

 Irre<?ulai-ities in growth may arise from the sponge receiving more nutriment 

 on one side than on the other, and differences in the aspect of individuals 

 belonging to the same species are found to be due to the fact that some 

 have grown in quiet waters and others where the water was in inotion. 

 The existence of such variability, which is corroborated by living sponges, 

 can not fail to embarrass the discrimination of specific values among fossil 

 forms. The Dictyosponges present some interesting examples of such 

 poJytrope species. The Jlydnoceras tuherosti'm, Conrad, is a vase-shaped, 

 sub-prismatic sponge, Avith several horizontal rows of strongly tufted and 

 pointed nodes, and all Dictyosponges having these characters might 

 very conveniently be referred to this specific type. We know, how- 

 ever, the location of several plantations or colonies of these sponges 

 on the sea-bottom of the Chemung period, and have endeavored to 

 make it apparent in the ensuing descriptions, that in each of these colonies 

 the individuals have an expression which characterizes the members of that 

 colony alone. These are distinctly local expressions, due to restricted 

 variability, and they serve to indicate in several instances, the precise spot 

 where they have grown. 



Individmdity. Sponges may assume either a simple or a compound 

 form ; growing as a single sac which never branches or divides, or as a bush- 

 like stock increasing its branches by the production of buds. The compound 

 mode of growth is rare among the siliceous sponges and there is no satisfactory 

 e\ndence that in the family DicTYospoNGiDiE there was ever a tendency to 

 duplication of the sponge by budding. The species Madodictya oscidata, of 

 the Keokuk shales, requires further investigation in this respect, as it is not 

 ■well understood. But as a whole these ancient sponges have rigorously 

 maintained their individuality, so far as that term is permissible in application 

 to a single skeletal structure. 



Mode of attachment. The vast majority of all sponges, recent and extinct, 

 have a portion of the body more or less distinctly specialized for fixation. 

 This attachment may be to solid extraneous bodies, rocks, pebbles, dead or 

 living shells, or may be effected by the simple penetration of muddy or sandy 

 sediments by the hnig, stout fibers of the skeleton. 



