PORIFERA 105 



without its siliceous cap or anchoring root-tuft. After the 

 decomposition of its soft parts, sediment very readily oozed 

 through the meshes of the skeleton, completely filling it. Upon 

 this sediment were impressed the form of the skeleton and the 

 shape of its quadrate meshes. With the disappearance of the 

 hydro-siliceous skeleton through the destructive agency of 

 alkali-bearing percolating waters, the shape of the mass of sedi- 

 ment within the skeleton remained. 



Prismodictya belongs to the family Dictyospongidas (Silurian- 

 Mississippian). These, unlike their modern representatives, 

 — Venus' flower basket, lived in comparatively shallow water. 

 Most of the Devonian forms, as Hydnoceras, were anchored in 

 sand or sandy muds by their root-tufts, while in the Mississip- 

 pian the majority were apparently fastened to a solid object, — 

 stone or dead shell, by a more or less broadened base ; this last 

 was probably also the case wdth Prismodictya. Ornamental 

 ridges may also occur {e.g. Clathrospongia and Thysanodictyon) , 

 upon the outer surface as in the modern Euplectella. 



Prismodictya prismatica is very abundant in the upper 

 Devonian (Chemung) sandstones of the Appalachian region. 



1. Sketch side view of entire specimen with a small portion 

 in detail, noting the quadrate meshes. 



2. Describe the steps in the formation of this fossil from the 

 death of the animal to its present appearance. 



3. How did the living Prismodictya differ from Venus' 

 flower basket ? 



Order 2, Tetradinellida. — Skeleton of four-rayed siliceous 

 spicules (whence the name from Greek tetra, four, + aktis, 

 ray, + Latin dim. ella). When these spicules are united with 

 each other, it is through the addition of spongin. These are 

 known in the fossil state from the Cambrian to the present. 



Astylospongia (Fig. 38). Ordovician-Silurian. 



Spherical, with a shallow depression (osculum) at the sum- 

 mit, and with many pores (incurrent canals) over the entire 



