188 DiCTYOSPONGIDiE. 



spicular bands, one broad and conspicuous, its members alternating with a 

 series of narrow bands. In the type-species, P. Dawsoni, where all generic 

 features are most clearly defined, these bands are generally placed at ecjual 

 intervals, but there may be in other cases, and even in this species, consider- 

 able variation in this respect. By the crossing of the concentric or horizontal 

 spicular bands, Avhich are narrow, of equal breadth and nearly equidistant, 

 the area enclosed by any two of the broader vertical bands is divided into 

 subequilateral quadrules, and each of these (piadrules is again divided into 

 four (piadrules by the intersection of a vertical band of the second order Avith a 

 horizontal band. Of the foiir quadrules thus formed two are elevated into 

 bullate expansions or pouches, the other two are depressed into regularly con- 

 cave basins; nodes and depressions alternating in position so that normally 

 every node is surrounded by four depressions and every depression by four 

 nodes, each at the same time being separated from every other by spicular 

 bands. Considerable irregularity in the disposition of these elevated and 

 depressed areas is sometimes shown by P. Colletti, while the little known P. 

 mi(ltib)trsarki in this structure quite fails to conform with our conception 

 of the genus. 



As usually preserved the tops of the surface nodes are broken off, or by 

 adherence to the matrix give the impression of a vertical series of areoles, but 

 normally the surface of these nodes and depressions is continuous and finely 

 reticulated by regularly disposed interlacing pentacts. All specimens 

 observed have been more or less subjected to compression and it is possible to 

 ascertain the full elevation of the nodes only along the edges of the cup. In 

 such marginal sections we also find evidence of extensions from the horizontal 

 spicular bands into slender erect tufts. These always occur at the intersec- 

 tion of the horizontal and vertical bands, and it is probable that they Avere 

 developed at every such intersection. The spicules of the vertical bands do 

 not appear to have been complicated with those of the horizontal bands in the 

 formation of these tufts.* 



There is certainly a striking similarity in external structure between 

 Physospongha. and the great explanate or saucer-shaped bodies already, 

 described as HYPHAm'^NiA, and which occur in the form of impressions in the 

 sandstones of the Chemung group. Our knowledge of HypHANTiEisriA is still 

 incomplete but we know that the intervals between the vertical and hori- 

 zontal or concentric spicular bands were partially if not wholly covered by 

 spongin. 



* la the brief ori^loftl description of tbU genua these tufts were regarded as originating from the summits of the nodes. 



