Genus of Fossil Hexactinellid Sponges. 15 



siliceous matter. Between the open meshes of this most 

 exquisite net (which, in the delicate and complex tracery of 

 its transparent fibres, surpasses almost every thing I have seen 

 amongst the Hexactinellida3) one observes either an intermesh 

 perfectly open and leading to the interior of the skeleton, or 

 else a multitude of minute glistening fibrelets, which pass 

 from fibre to fibre of the secondary rete below, and weave 

 across its meshes a transparent vitreous web (PL V. fig. 3). 

 The secondary rete passes in its turn into the skeletal network 

 below, which, at first beautifully spined, soon becomes, as it 

 leaves the surface, perfectly smooth. 



Tlie foregoing facts could be observed by examining the 

 surface of the etched fossil by reflected light; but by splitting 

 off a fcAV fine chips with a scalpel, treating them with acid 

 in a watchglass, washing with distilled water, and finally 

 drying, the network could be obtained in a state fit for 

 mounting in Canada balsam and other media, and for obser- 

 vation with transmitted light. 



Traces of the network with complex fibres may be detected 

 along the lateral edges of the oscular plate in the specimen 

 where it occurs ; but further on, over the anterior face, it quite 

 vanishes, and only the ordinary superficial reticulation prevails 

 (PL V. fig. 4). 



Flesh-spicules. — The perfect manner in which the super- 

 ficial network is preserved led me to think that some rosettes 

 or other flesh-spicules might perhaps be seen in the sponge; 

 and the most likely places to look for them appeared to be, first, 

 in the residue set free in suspension on treating the fossil 

 with acid, and, next, in the open meshes of the skeleton. A 

 careful examination of the former proved altogether unsuccess- 

 ful, while in the latter iron pyrites was observed under a 

 variety of forms. In this there was hope, since I have slides 

 showing minute coccoliths and delicate radiolarians perfectly 

 preserved in this material : therefore I made a long search in 

 the expectation of finding some form of iron pyrites which 

 should display evident traces of the rosette form ; but, with a 

 few very unsatisfactory exceptions, my search was quite in 

 vain. The flesh-spicules of the Hexactinellid^ have yet to be 

 found in the fossil state. 



Other Sjncules. — I have, however, met with two spicules 

 other than sexradiates in this fossil. One is a completely 

 erectly spined cylindrical form (PL V. fig. 5) with one 

 part hidden in the network, from which the other portion 

 projects freely, making an acute angle with the oscular sur- 

 face as it points upwards from the base. This spicule bears 



