98 R. Kirkpatkick: A New Variety of Spon. Xoricata.. [Vol.11, 



Description. — The upper surface of the sponge is coarsely 

 prickly and spinous, and with numerous small cylindrical chim- 

 ney like oscules rising vertically above the general surface to a 

 height of about 6 mm., the diameter at the orifice being from 2 to 

 2*5 mm. ; the outer wall of the oscules is provided with longitu- 

 dinal spinous ridges. 



The spines are either single and pointed, and from i to 2 mm. 

 in height, or broader, and divided only near the summit; or 

 again they may be united into small ridges with slender prickles 

 along the edges. 



The under or encrusting surface is level ; here and there are 

 genimules isolated or in groups ; and over considerable areas is a 

 reticulate pattern, each mesh of the reticulum having a small 

 circular rim near its centre ; these markings are formed by the 

 front walls of the zooecia of the Polyzoon Hislopia lacustris, the 

 circular rims being the orifices of the zooecia ; the posterior walls 

 of the zooecia have been left behind on the wooden beam. 



The dermal membrane is firmly attached, and there is not 

 a difterentiated dermal skeleton. The dermal membrane roofs 

 over a fine surface reticulum, the meshes of which are about 2 to 

 •4 mm. in area ; groups of these fine meshes are often sunk in 

 concavities between groups of the larger spines ; the pores are not 

 visible. 



Skeleton. — A thin vertical section of the crust in balsam 

 shows vertical pillars continued up as spines. The pillars, which 

 vary from '2 to '4 mm. in thickness, are from i to 2 mm. apart ; 

 they do not arise from the base, but at a point a little above the 

 base, or even half way up ; the section shows, also, traces of hori- 

 zontal lines of cleavage, marking possibly stages of growth or 

 temporary arrest of growth. 



Between the vertical pillars is a fine reticulum with meshes 

 rounded or oval in section, and about '2 to '6 mm. in diameter, 

 the strands being about 2 to 7 spicules thick. The gemiiiules are 

 situated in the encrusting base of the sponge. They are either 

 single and spheroidal and about "8 mm. in diameter, or double and 

 oval and about i X i'3 mm. in diameter. There are three layers— 

 an outer shell of large smooth strongyles similar to those of the 

 general skeletal framework, a middle layer of finely spined micro- 

 strongyles. and a thick, inner chitinous coat ; the middle layer again 

 is divided into two — an outer one applied to the inner surface of the 

 outer shell, and an inner one embedded in the chitinous shell. 



The poral tube (fig. 5), best seen in specimens which have been 

 desilicified with hydrofluoric acid, is nearly cylindrical and 45 m 

 in length. ' 



Spicules, megascleres, strongyles (fig. 6), sinooth, slightly 

 curved, occasionally a little swollen at the ends, 270 X 195 on an 

 average ; very commonly with one or more fusiform swellings. 



Microscleres amphidisks (fig. 7), numerous and scattered about 

 in the flesh, 30 — 45 .m long, with slender curved smooth shaft, and 

 with 4—5 prongs to each disk. 



