6 Mr. W. J. SoUas on Stanronema, a neio 



that in a specimen 1^ inch high, with an oscular plate uni- 

 formly i inch in thickness throughout, it has increased from 

 a mere trifle at the base and the edges to i inch at the top and 

 through the middle of its face. In contrast also with the 

 uniform character of the oscular plate is the irregularity of 

 growth manifest in this portion : in one class of forms it 

 increases in a series of bulgiiigs, Avhich form g-ently rounded 

 swellings concentric with the distal margin, or rounded ridges 

 so regular as to give the hinder surface a corded appearance ; 

 sometimes the gentle swellings are not continuous but sink 

 laterally into faint dimples ; while the ridges are not always 

 semicircular, but occasionally change their coiu'se abruptly so 

 as to be V-shaped at one side. 



Above, the upper surface of the posterior mass may be 

 gently rounded against the oscular plate, or it may form a 

 flat table and join the plate at right angles. 



Underlying the variations in this class of forms there is, how- 

 ever, a certain degree of regularity ; in all the posterior mass 

 extends laterally as far as the oscular plate, and the two are 

 conterminous along the lateral edges, whilst above, whether it 

 joins the oscular plate gradually or abruptly, it always follows 

 the general curve of the latter in a simple or nearly simple 

 line. But in another class of forms, which, I think, constitute 

 a separate species, the irregularities are much greater than 

 the foregoing ; in them the posterior mass is seldom ridged 

 concentrically, but soon after leaving the base it becomes 

 lobed vertically into two or more diverging processes, differing 

 in size and shape, and exposing the oscular plate in the angle 

 between them : in these forms the posterior mass reaches 

 the lateral margins of the sponge near the base only, and soon 

 ceasing to do so as it ascends, allows the anterior plate to extend 

 freely beyond it in a lateral as well as in a vertical direction. 



Externally the porous mass presents a plain surface, never 

 excavated by oval pits or specialized pore-areas. In section it 

 exhibits a number of canals, which, passing from the interior 

 in a more or less wandering course, and without any regular 

 arrangement, terminate at length against the attached face of 

 the oscular plate, into the excurrent canals of which they in 

 some cases directly open ; but whether they do so always seems 

 to me doubtful. 



Minute Structure. — To investigate this the fossil may be 

 prepared in two ways : it may either be treated with some 

 acid (I prefer nitric) by which the matrix of calcite is readily 

 dissolved, while a siliceous network is, in well preserved spe- 

 cim.ens, left in relief ; or slices may be cut from it and ground 

 down till thin enough to be transparent ; this is the method 



