Genus of Fossil HexactinelUd Sponges. 11 



shows that the sexradiate forms are in all probability produced 

 bj a radiate growth from the first of the six arms from a 

 common centre : this being so, one can readily see that if the 

 growth of a free radius took place in the course of the excur- 

 rent canal, it would be subject to a pressure in two directions 

 at right angles to each other — one due to its growth onwards, 

 normal to the surface from wliich it spi'ings, and the other 

 parallel to the axis in the direction of the current ; and its 

 ultimate position would be the resultant of these two, and 

 would be in just such a position as the spines, in fact, assume. 



The growth of the spicule from a mother cell also explains 

 in part many other matters which would otherwise be enig- 

 matical. Thus the wonderful regularity of the network we have 

 previously described may be looked upon as having resulted 

 from a motlier cell which originally gave off buds, one at the 

 end of each of its spicular rays — i. e. in the direction of most 

 active growth ; the cells so budded off would become in turn 

 mothers, and repeat the process, till, by reason of the limita- 

 tions imposed by the limits of the organism, they would be 

 unable to produce more than onebud each, and that vertically — 

 except that when the distance between two cells became much 

 greater laterally than twice the length of a spicular ray, a 

 fresh cell would thus appear at the side of one of them, and 

 the vacant place be filled up. 



Detached Oscular Plate. — The thin plate of sponge-struc- 

 ture mentioned on ~p. 4 is bounded on all sides but one by 

 a broken edge ; the edge which is not broken is one of the 

 lateral margins, neatly rounded off in the same way as are 

 the sides of the oscular plate in Stauronema (PI. I. fig. 9, 

 n n n). Anteriorly the plate is marked by oscular pits 

 (fig. 9, a) quincuncially arranged, and of the same shape, size, 

 and distance from one another as in Stauronema. These pits 

 are the mouths of cylindrical excurrent canals, which perforate 

 the plate and open posteriorly in rounded pore-areas. The 

 structure intervening between the pore-areas is frequently raised 

 into ridges and prominent monticules, more marked than those 

 which occur on the posterior surface of Stauronema, but 

 otherwise similar ; the skeletal networks of both fossils have 

 also the same structure and arrangement ; and their meshes 

 and fibre are of the same dimensions. These facts, and the 

 absence of the true distal margin of the oscular plate in the 

 other specimens, leave little doubt in my own mind as to the 

 relation which this fossil bears to the latter. I cannot but 

 regard it as a part of a distal expansion of the oscular plate of 

 Stauronema. 



Posterior Mass. — Between the canals of the posterior mass 



