134 ART. 7. — I. IJJMA : IIEXACTINELLIDA, IV. 



In the Acanthasciiue the basidictyonal plate is never of any 

 considerable thickness, but always thin and insignificant. The 

 elements {basidictyonalia) composing it are, as usual, small but 

 comparatively thick-rayed hexactins — sometimes spicules with a 

 less number of I'ays — in which the rays are roughend by the 

 presence of microtubercles either all over the surface or near the 

 ends only. The spicules are both directly and synapticularly 

 fused with one another, formino- a ri<iid framework with irresiular 

 meshes, though there may occasionally occur such as are yet 

 unfused or are in the process of fusing together. The uneven 

 limiting surface presented by the plate against the substratum is 

 covered by a special, thin and sieve-like siliceous layer perforated 

 by very small meshes {vide esj)ecially PI. XXL, fig. 12). This 

 limiting reticular layer, which may be regarded as a part of the 

 basidictyonal plate or mass, was long ago noticed and figured by 

 F. E. Schulze (Chall. Eep., PI. LXIV., fig. 3). It is a 

 structure that reminds one of the " Deckschicht " that is known 

 to cover the exterior of many fossil Dictyonina. Occasionall}'- fine 

 axial canals in the form of a plane cross are found inclosed in 

 its beams (PI. XXII., fig. 17), indicating that a stauractin, 

 which may be classed under the basidictyonalia, is structurally 

 involved in it ; but by far the greater part of the la3^er consists 

 of synapticula-like deposits formed in connection with the l)asidic- 

 tyonalia present and invariably in direct relation with the foreign 

 bod}^ with which the s2)onge-base is in contact. It sometimes 

 happens on fragments of the limiting layer taken from certain 

 species or individuals, that no basidictyonal spicules in union with 

 the beams can be seen. This mav be due to their beini>; somehow 

 concealed from view, perhaps in that they are too sparse and 

 widely scattered to be easily discovered. At the same time I 



