iNTRODtlOTOBY. 1 9 



ment to the sediment in the form of a minute anchor at the extremity of each 

 rod. Such root-spicules are not always present but it would seem that 

 in most of the Dictyospongid^ they have been more or less highly 

 developed. About the aperture or osculum there may also be a series 

 of prostalia, known as the marginalia, which may be present or absent in the 

 thin-walled sponges. At intervals over the lateral surface of the sponge there 

 may be tufts of lirostalla, such as frequently occur in the Dictyospongid^ ; 

 these have been termed plewalia. The Dictyospongid^ in common with 

 the closely allied family Euplectellid^, also possess horizontal and vertical 

 series of smooth rods in bundles of greater or less size. These cross each 

 other at right angles and produce a reticulated surface Avhich is a leading 

 characteristic of the entire group. The regularity and uniformly descending 

 order of this reticulation is a marked character of these sponges. The regu- 

 larity of the meshes is, indeed, a feature shared with the Pkotospongid^, but 

 in that family there is no such nmltiplication of the meshes by subdivision as 

 occurs in the Dictyosponges. Though this gradually descending subordinate 

 reticulation is seen in all true members of this family, it has not been deter- 

 mined that the descending series of intersecting spicular bundles which produce 

 the mesh-work are always of a definite number. If, in non-prismatic species, 

 the unifonnly largest prevailing quadrule be fixed upon as that produced by 

 the primary spicular bands (Thysanodictya, Clathrospongia) or, in prismatic 

 fomis, by the bands lying on the prism angles and intersecting Avith equi- 

 distant horizontal bands (Prismodictya, Hydjioceras), it is found that 

 there is a sub-division of the primary quadrule, or Tnere, to the fifth 

 degree; that is, there is a descending series of six sets of reticulating 

 bands, forming quadndes which, in terms of the primary (piadrule, or 

 mere, may be designated as dimeres, trimeres, Utrameres, pentameres and 

 hexanieres. Oidy in rather unusual instances of delicate preservation are 

 the ultimate quadrules, or hexaraeres, retained. Such reticulation is a quite 

 uniform cliaracter of the majority of the genera of these sponges, but 

 there are instances, seen in such genera as Piiysospongia, Roemerispongia 

 and Hypiiant^nia, in which the uniformity of this gradation is 

 lost ; and Avhere there is a single or double series of major quadrules 

 whose surface is covered by an exceedingly minute reticidation 

 (Piiysospongia) or, possibly, by none at all (Roeiierispongia, Hypiiant^nia).* 



'The fine reticulation of tlie major quadrules in Piiysospongia. seems, as elsewhere observed, to bo 

 due not to intersecting sincular threads or bundles, but largely to regularly arranged hexiictios of thf 

 parenchyma; while there are good reasons for regarding the apparently uncovered (luadrules in Roemer- 

 I8P0NGIA and UvpiiANTiENiA ;!8 due to the imperfect retention of this delicate layer. 



